We offer a full pond filtration installation and maintenance service. (Click Here) If you want to choose a filtration system yourself here is a guide to the basics.
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This fact sheet outlines the basic principles of pond filtration and some of the more common types of filter. It sets out to give the reader a good grounding and does not attempt to be fully inclusive.
A good filtration system will stop the pond going green and it will improve the health and clarity of the water. It is quite possible to have a clear healthy pond without a filter, but without one you can never be sure of success. If you want to be certain that your pond water is going to be clear and confident that the fish will be healthy then you are probably going to want a filter.
There are three main reasons to filter a pond.
To prevent the water going green
To keep the water healthy
To keep the water clear
To prevent the water going green
by killing the single-celled floating algae that cause green water. Most filtration systems will include an ultra violet clarifier whose purpose is to kill the algae. The ultra violet clarifier will usually be the first stage of filtration.
To keep the water healthy
by rendering toxic waste products produced by the fish (ammonia & nitrite in the form of urine and faecal matter), harmless by the biological action of beneficial bacteria (nitrosomonas & nitrobacter).
The nitrogen cycle - fish release ammonia into the water as a by-product of their metabolism and digestive processes. By a process know as nitrification this is converted by the naturally occurring nitrosomonas bacteria into nitrite. Nitrite is much less toxic to the fish than ammonia but it will still poison your fish if too much is present in the water. Nitrobacter another naturally occurring bacteria will convert the nitrite into nitrate. Nitrate still has a low toxicity but this is much less than nitrite and the concentration of nitrate necessary to harm your pond fish is much higher than is likely to occur naturally in a pond. At each stage of the nitrogen cycle the nitrifying bacteria oxydize the pollutant reducing its toxicity. The final product nitrate is effectively harmless and a powerful plant fertiliser.
The bacteria involved in this process do not float about freely in the water; they need a physical surface to grow on. The main body of the filter will be largely filled with filter media placed there for the bacteria to grow on.
Filter media is the term applied to materials placed in the filter for bacteria to grow on. Filter media is a general term including many different materials. The larger the surface area of the filter media, the greater the number of bacteria that can live on it and this increases the amount of fish waste that can be digested and made safe by the bacteria. All filter media has a large surface area and is designed so that water can flow freely through it easily reaching all the bacteria. Types of filter media include foam, brushes, plastic media (including chopped up tubing - flocor), lava rock, industrial slag, gravel and clay granules.
The beneficial bacteria need well oxygenated water. Heavy clogging with mud can cause oxygen starvation to parts of the filter and a reduction in performance. If this happens a different kind of bacteria that don't take their oxygen from the water will take over. These bacteria are bad for the health of the pond and will produce waste by-products harmful to the fish.
To keep the water clear
Pond water will often contain mud (organic matter, fish excrement, soil, dust blown into the pond and other debris) suspended in the water as fine particles. A good filter will remove these suspended solids creating a greater clarity of water. Dead algae killed by the ultra violet clarifier are also removed in the filter.
In a pressure fed or gravity fed filter, the first chamber before the biological part of the filter will usually be a settlement chamber. An effective settlement chamber will reduce the amount of mud getting through to the later stages of filtration where it might block the filter media and reduce the performance of the filter. A simple settlement chamber is there to reduce the turbulence of the incoming water and to create stillness. As more turbulent water becomes still some of the suspended particles of debris contained in the water will drop to the bottom and settle out of suspension. A settlement chamber may contain pond water and nothing else but more commonly it will contain filter brushes. These assist in creating still water and trapping suspended particles.
A more sophisticated filter might have a vortex settlement chamber as its first stage. This a another way of removing suspended particles. Vortex settlement consists of a single large circular chamber. Water enters this chamber at an oblique angle causing the water to spiral. This spiralling causes the suspended solids to migrate to the bottom of the spiral, to the bottom of the chamber settling out as a layer of mud on the floor.
Most filters will include some form of mechanical filtration. In this context this does not mean a machine with working parts. It simply means a non-biological way of improving the clarity of the water. Foams and brushes in addition to their biological action will trap solid particles of debris carried into the filter by the water. The first chamber in a filter will commonly include brushes. The last chamber in a filter will commonly include foams. The filter foams in the final chamber of a filter are usually included with three or more sheets of increasing fineness to catch very small particles. The three layers of foams in the final chamber are there to give a final polish to the water and remove any last remaining particles that the previous chambers of the filter have failed to trap. Large multi-chambered filters may also use foam blocks (not sheets) and/or brushes in one or more of the middle chambers for the cultivation of filter bacteria.
A filter will allow you to have more or larger fish in your pond. A filter will allow you to feed the fish more food or more often. A filter will prevent the pond going green. A filter will achieve greater clarity of water.
A filter will not prevent mud building up in the pond (its not a hoover). A filter will not prevent blanketweed growing in the pond A filter will not remove the need for regular clean outs. A filter will not remove the need for regular maintenance of the pond. A filter will not allow unlimited stocking of the pond with fish. A filter will not allow unlimited feeding of the fish. A filter will not eliminate the need for the water to be tested regularly. A filter cannot be guaranteed to solve brown water problems.
If a filtration system is to function properly it must be switched on all the time and never turned off (except briefly to allow maintenance work to take place). This allows the necessary bacteria to live and grow in the filter. It takes months to build up a mature bacterial colony in a filter. Switching off the filter shuts off the supply of food and oxygen to the bacteria and they die. Without the bacteria you have lost all the biological filtration and half the functionality of the filter. The ultra-violet clarifier must be kept switched on all the time too if it is going to kill all the algae that turn the water green and prevent it coming back.
When the filter is first installed it should be able to keep the water clear but it will have a limited beneficial effect on the health of the pond water. A new filter is incomplete and not fully functioning. There will be none of the bacteria present necessary to digest the waste products produced by the fishes. The establishment of a bacterial colony can take as long as 18 months to fully mature. It is important to take particular care not to over feed the fish in the first weeks of a new filter and to test the pond water for ammonia and nitrate more frequently.
It is much better to buy a filtration system that is more powerful than you need than to buy something that it not up to the job. Your filter should have spare capacity to give you a good margin of safety. It should be able to cope as your fish increase in size.
There are certain factors which will place additional demands on a filtration system. These need to be taken into account when specifying a filter. Koi are much heavier polluters than goldfish and other small fish. Most filters manufacturers will publish a maximum pond size for each of its filters. It is sensible to reduce this maximum recommended size by half if you plan to keep a moderate number of koi. If you are planning to heavily stock your pond with koi you will want to reduce this figure further.
A shallow pond containing a 1000 gallons will need a more powerful ultraviolet clarifier than a deep pond containing 1000 gallons. The deep pond will of course have a smaller surface area than the shallow pond if they both contain the same amount of water.
A pond in full sun will need a more powerful ultra violet clarifier than a pond in shade.
Most manufacturers will publish a table giving the maximum sized pond for each filter and the effect these different factors will have on the performance of their filters.
A conventional pump fed filtration system will usually consist of these parts:
A Pump This supplies the water to the filtration system under pressure.
An Ultra Violet Clarifier This prevents the water going green.
A Settlement Chamber This slows the water and allows solids to drop out of suspension.
Biological Filtration Bacteria break down the waste produced by the fish and make it harmless.
Mechanical Filtration Traps solids and removes them from the water.
Pipe work - This connects the parts of the filter together. This will consist of inlet hose taking water from the pond to the filter and outlet pipe taking water that has passed through the filter back into the pond.
The main purpose of this unit is to stop the pond going green. The water passes through the U.V.C lamp, which subjects the water to a strong dose of Ultra Violet light. The Ultra Violet light disrupts the DNA of the algae which turn the water green, and causes them to die.
The box filter will contain the filter media needed to promote the biological filtration of the pond water. The bacteria which grow on the filter media digest the bodily wastes (ammonia & nitrite) produced by the fish and convert them in to harmless nitrates. The filter will also remove fine particles of mud suspended in the water. Dead algae killed by the ultra violet clarifier are also removed in the filter.
At their simplest these are usually a simple rectangular box containing lots of small filter media such as chopped up plastic pipe (flocor). On top of this will usually be three layers of reticulated, cellular foam (reticulated - an evenly dimpled surface a little like the inside of an egg box), (cellular - the air pockets link together to allow liquid to flow freely through the foam).
A multi-chamber filter will contain separate chambers (usually between two and six chambers), each chamber containing a different form of filter media. In a filter with three or more chambers the first chamber will probably be a dedicated settlement chamber designed to slow the incoming water down and remove suspended mud particles. The final chamber will probably contain some small loose filter media and three layers of foams to polish the water, removing any remaining suspended solids. Any chambers in between are likely to contain a variety of filter media dedicated to growing the bacteria necessary for the biological filtration of the pond water.
Multi-chamber filters come in a variety of configurations. The simplest are a long rectangular box. The box is divided into separate compartments (chambers) at regular intervals by panels. The water is channelled through the media in an over an under system. If the filter has a vortex settlement chamber, the water will usually enter the filter near the bottom. The dividing panel between the first and second chamber will be low at the top allowing the water to flow over it into the top of the second chamber. The water will then flow downwards through the media in this chamber. The next panel will have a gap at the bottom and will be high at the top. This takes the water from the bottom of the second chamber into the bottom of the the third chamber allowing the water to rise up through the media in the third chamber. Water continues to flow through the filter zig zagging from chamber to chamber. Up through the media in one chamber down through the media in the next.
If the water is fed into the first chamber through a spray bar this will mean the water is coming in at the top of the filter. The route through the chambers will different than where the water comes in at the bottom (as in a filter with a vortex settlement chamber) but it will still be an under and over zig zag.
Another common filter type of filter is made up of round barrel shaped chambers joined together. These may be joined in a straight line or configured in a square producing a four leaf clover pattern. Water will be introduced from one chamber to the next at an oblique angle setting up a gentle spiralling flow of water in each of the chambers. The bottom of a barrel will usually come to a point producing a cone shape at the base. Mud will collect at the point of the cone and this can be removed from the filter by means of an opening at the point of the cone controlled by a valve.
A submersible pump is located in the pond. The box filter will be located close by the pond in the garden. The box filter should be installed so that it is level. Water is very heavy. Large filters should be placed on a firm level concrete base.
The location of the pump should be as far away as possible from where the water comes back into the pond after it has gone through the filter. This ensures that all the pond water is circulated through the filter. (If you locate the pump close to the filter outlet, much of the water returned to the pond from the filter will be quickly pumped straight back into the filter and water from further away will be left untreated.)
Water is forced under pressure by the pump through a single inlet pipe into the ultra-violet clarifier. The ultra-violet clarifier is connected to the main biological filter by another single pipe. This water is under high pressure and so these pipes are smaller than the size of the pipes returning the water under low pressure back to the pond. Once the water enters the filter it is no longer being moved along by means of the action of the pump. The water then moves under its own weight. As the filter is filled with water, the level rises until the outlet pipes at the far end of the filter are reached. The filter water will overflow through these outlet pipes back into the pond. The water exiting the filter will return to the pond under its own weight through the action of gravity. For this reason it is important that the level of the filter outlet pipes is higher than the surface of the pond. For the same reason it is important that the pipes from the filter back to the pond are set at a continuous fall. There must be no 'U' shapes in the filter out let pipe. The water must never be expected to flow uphill. The outlet pipes will commonly be a large single pipe or on a large filter two large pipes.
If the filter isn't very close to the pond you will need to buy additional pipes to take the water from the filter back to the pond. Most filter manufacturers will only supply rigid pipes with their filters, so that it helps if your filter outlet pipes are in a direct line with the pond. If this is not possible 90 degree and 45 degree bends are usually available but this does not give you much freedom. You will still need to position your filter carefully. If your filter has return pipes 2 inches or less in diameter then you can use flexible hose instead of the usual rigid pipe. Using flexible hose will allow you much greater freedom in positioning the filter. It is also easier to bring the filter pipes back to the pond discretely if they are flexible. For example if the pond has a rock garden, the pipes can be brought in between two rocks or hidden in the pebbles of a beach. It is much harder to do this if the pipes are rigid.
The bacteria in the filter take oxygen from the water. It is a good idea to return the water to the pond in a way that causes it to splash. This helps it to replace the lost oxygen.
Over time mud will accumulate throughout the filter. Excessive mud will reduce the performance of the filter. Blocked brushes and foams will reduce the filters ability to remove further mud and suspended solids from the water. Clogging of the filter media can reduce the water flow through the media and cause a reduction of oxygen and food to bacteria living in parts of the filter. This may cause bacteria in the filter to die reducing the ability of the filter to process the waste produced by the fish. Some smaller filters and most of the large filters have valves allowing you to drain down the filter. Either a single valve for the whole filter or individual valves one for each chamber.
Foams and brushes should be cleaned once a month. Foams should be squeezed out in a large container holding water taken from the pond. Filter brushes should also be rinsed in pond water and shaken out. Tap water should never be used. It will kill the bacteria living on the filter media reducing the performance of the filter. If the water gets muddy replace it with more clean water from the pond. The filter media should be fairly clean but it doesn't need to be obsessively clean.
The other media will need cleaning too but providing the pond is cleaned out regularly (once a year is good) it will be unnecessary to clean it as often as monthly. This can be done once a year. For best results do it at the same time as the ponds annual clean out.
The bulb contained in the ultra violet clarifier will need replacing every so often. Usually (although this is certainly not true for everyone) the bulb will last at least a year. If the pond goes green change the bulb. Otherwise change the bulb annually or if you prefer wait until the water starts to go green before you change the bulb.
The bulb sits inside a crystal tube. This keeps the water away from direct contact with the the bulb and the electrical connections. The crystal tube allows the light to come through from the bulb into the water where it kills the algae. Lime scale can build up on this crystal tube blocking the light from the bulb and reducing the performance. When this happens the lime scale must be removed, if the ultra violet clarifier it to work at full performance. Soaking the crystal tube in white vinegar will remove the lime scale.
A good correctly specified pump fed filter is a very effective way of keeping your pond clear and healthy. They have a few imperfections in their design. All the water is pumped into the filter. As it does this the pump churns the water chopping and spinning it rapidly through its impeller. This is not ideal. It makes the task of removing the suspended solids more difficult (A gravity fed filter is designed to avoid this. The water passes into the filter gently and smoothly).
The filter media must be cleaned occasionally by hand. This is a dirty messy business.
If the filter should become blocked (commonly by filter foams silting up or leaves blocking the outlet pipe) the water level can rise in the filter and overflow onto the garden and be lost. If the filter overflows and nobody notices this (perhaps because they are away on holiday, at work, or asleep) the pond can empty, perhaps completely causing the deaths of the fish. This can happen but it is uncommon and if it does happen it is usually caused by neglecting maintenance on the filter.
Cannister filters are much smaller and more discreet than conventional filters and they are easier and cleaner to maintain.
A pressure fed cannister filter will usually consist of these parts:
A Pump This supplies the water to the filtration system under pressure.
An Ultra Violet Clarifier This prevents the water going green.
Biological Filtration Bacteria break down the waste produced by the fish and make it harmless.
Mechanical Filtration Traps solids and removes them from the water.
Pipe work - This connects the parts of the filter together. This will consist of inlet hose taking water from the pond to the filter and outlet hose taking water that has passed through the filter back into the pond.
Cannister filters are small circular sealed containers that can be sunk into the ground. They consist of a biological filter usually containing foams with an ultra violet clarifier in the lid. Unlike a conventional filter they do not have a settlement chamber.
Water is pumped into the filter under pressure of the pump and because they are a sealed container the water that comes out of the filter is still under pressure. This means a smaller return pipe can be used than would be needed if a conventional filter was being used. This high pressure water going back to the pond also means that a canister filter can be installed in the ground at a lower level than the water level in the pond. This is especially useful if you have a raised brick pond.
The ultra violet clarifier in not a separate unit apart from the main filter unit. It is contained in the lid of the filter. The main purpose of this unit is to stop the pond going green. The water passes through the U.V.C lamp, which subjects the water to a strong dose of Ultra Violet light. The Ultra Violet light disrupts the DNA of the algae which turn the water green, and causes them to die.
The main body of the filter below the lid will contain the biological filter media. This will usually contain foams (Although some canister filters will contain other types of media mainly plastic or rocky media). The biological filter will contain the bacteria which will digest the bodily waste produced by the fish. The filter will also trap dead algae killed by the ultra violet clarifier and mud particles in suspension in the water.
A submersible pump is located in the pond. The pressurised filter will be located close by the pond in the garden. The filter is dug into the garden so that only the top two or three inches are above ground.
The location of the pump should be as far away as possible from where the water comes back into the pond after it has gone through the filter. This ensures that all the pond water is circulated through the filter. (If you locate the pump close to the filter outlet, much of the water returned to the pond from the filter will be quickly pumped straight back into the filter and water from further away will be left untreated.)
Water is forced under pressure by the pump through a single inlet pipe into the ultra-violet clarifier located in the lid of the filter. Once the water has passed through the ultra-violet clarifier it will pass into the main body of the filter containing the biological filter media usually these will be foams. From here the water will pass out of the filter through a single outlet pipe. Unlike other filters because the filter has a sealed lid, the outlet water is still under pressure from the pump. For this reason the filter can be located at a lower level than the pond and the outlet water can be directed uphill or by a circuitous route back to the pond.
The bacteria in the filter take oxygen from the water. It is a good idea to return the water to the pond in a way that causes it to splash. This helps it to replace the lost oxygen.
Over time mud will accumulate throughout the filter. Excessive mud will reduce the performance of the filter. Blocked foams will reduce the filters ability to remove further mud and suspended solids from the water. Clogging of the filter media can reduce the water flow through the media and cause a reduction of oxygen and food to bacteria living in parts of the filter. This may cause bacteria in the filter to die reducing the ability of the filter to process the waste produced by the fish.
A pressurised cannister filter is much easier to maintain than a conventional filter. The best canister filters can be cleaned without opening them up. This is done by back washing. A pipe for the dirty water is connected to the filter at the place provided on the lid. A valve is turned and water is sent around the filter in reverse and out of the pipe and on to the garden. There is often a handle which you can pull which squeezes out the foams inside the filter without having to take them out. (This is why many canisters contain only foams, other media cannot be squeezed out in this way) You will need to pull the handle in and out slowly a few times. The water coming out of the pipe will probably be very dark and dirty. After a while the water will come out much clearer. When this happens the valve can be turned back so that the filter operates normally and the pipe for the dirty water can be disconnected and put away. This should be done monthly.
The bulb contained in the ultra violet clarifier will need replacing every so often. Usually (although this is certainly not true for everyone) the bulb will last at least a year. If the pond goes green change the bulb. Otherwise change the bulb annually or if you prefer wait until the water starts to go green before you change the bulb.
The bulb sits inside a crystal tube. This keeps the water away from direct contact with the the bulb and the electrical connections. The crystal tube allows the light to come through from the bulb into the water where it kills the algae. Lime scale can build up on this crystal tube blocking the light from the bulb and reducing the performance. When this happens the lime scale must be removed, if the ultra violet clarifier it to work at full performance. Soaking the crystal tube in white vinegar will remove the lime scale.
Canister filters have their limitations. They are only suitable for smaller ponds of up to about 1500 gallons and about half this size if you plan to have a moderate number of koi. This is still a good size. Most household ponds are smaller than this. There are some very good canister filters but many of them are not so good. Some are too fragile, lack performance or still involve opening up the filter and cleaning the dirty filter media by hand.
These filters are much smaller than their conventional counterparts. This is achieved largely by reducing the size of the biological part of the filter. This reduction must lead to a reduction in the performance of its biological function. Having said this I have installed many of these filters and I continue to be surprised at how good they are and how much they can do. If you don't have a large pond and if you have a moderate number of fish, a canister filter is worth considering. Their small size and easy maintenance makes them very attractive.
A Pump This pumps water out of the filter and into the pond.
An Ultra Violet Clarifier This prevents the water going green.
A Settlement Chamber This slows the water and allows solids to drop out of suspension.
Biological Filtration Bacteria break down the waste produced by the fish and make it harmless.
Mechanical Filtration Traps solids and removes them from the water.
Pipe work - This connects the parts of the filter together. This will consist of inlet pipe taking water from the pond to the filter and outlet pipe taking water that has passed through the filter back into the pond.
A gravity fed filtration system is considered to be the most sophisticated and superior type of filtration for a pond. They are very similar to a conventional pressure fed filtration system. They have the same type of ultra violet clarifier, they have a multi-chamber biological filter containing filter media and a settlement chamber. The main difference is the way the water is transported into the filter. The water is not pumped into the filter from the pond. Water is instead pumped out of the filter. This causes a pressure reduction at the filter end of a pipe that connects the filter to the bottom of the pond. This pressure difference causes the water to flow from the pond into the filter along this pipe replacing the water as it is removed from the filter by the pump.
The main purpose of this unit is to stop the pond going green. The water passes through the U.V.C lamp, which subjects the water to a strong dose of Ultra Violet light. The Ultra Violet light disrupts the DNA of the algae which turn the water green, and causes them to die.
A multi-chamber filter will contain separate chambers (usually between two and six chambers), each chamber containing a different form of filter media. In a filter with three or more chambers the first chamber will probably be a dedicated settlement chamber designed to slow the incoming water down and remove suspended mud particles. The final chamber will probably contain some small loose filter media and three layers of foams to polish the water, removing any remaining suspended solids. Any chambers in between are likely to contain a variety of filter media dedicated to growing the bacteria necessary for the biological filtration of the pond water.
Multi-chamber filters come in a variety of configurations. The simplest are a long rectangular box. The box is divided into separate compartments (chambers) at regular intervals by panels. The water is channelled through the media in an over an under system. If the filter has a vortex settlement chamber, the water will usually enter the filter near the bottom. The dividing panel between the first and second chamber will be low at the top allowing the water to flow over it and into the top of the second chamber. The water will then flow downwards through the media in this chamber. The next panel will have a gap at the bottom and will be high at the top. This takes the water from the bottom of the second chamber into the bottom of the the third chamber allowing the water to rise up through the media in the third chamber. Water continues to flow through the filter zig zagging from chamber to chamber. Up through the media in one chamber and down through the media in the next.
Another common type of filter is made up of round barrel shaped chambers joined together. These may be joined in a straight line or configured in a square producing a four leaf clover pattern. Water will be introduced from one chamber to the next at an oblique angle setting up a gentle spiralling flow of water in each of the chambers. The bottom of a barrel may come to a point producing a cone shape at the base. Mud will collect at the point of the cone and this can be removed from the filter by means of an opening at the point of the cone controlled by a valve.
In a conventional pressure fed filtration system the water is pumped into the filter. As it does this the pump churns the water chopping and spinning it rapidly through its impeller. This is not ideal. It makes the task of removing the suspended solids more difficult. The gravity fed filtration system is designed to avoid this. The water is channelled gently into the filter from the pond under its own weight.
The main biological part of the filtration the multi-chambered filter box will be dug into the ground close to the pond, so that it is level or very slightly higher than the level of the water in the pond. There is an opening in the bottom of the pond connected to a very large pipe. This pipe connects the bottom of the pond with the first chamber in the filter. This way of connecting the filter to the pond means that the pond and filter water are effectively joined and part of the same body of water. The water cannot be higher in either the pond or the filter. If it is, the higher water will exert a greater pressure on the water in the connecting pipe. This greater pressure will cause the water to travel along the connecting pipe from high to low pressure until the water level and water pressure in the pond and filter are equalised.
The pump is located in the final chamber of the filter. Water is pumped out of the filter. This causes the water level in the filter to drop reducing the pressure of water at the filter end of the connecting pipe. This causes the pond water which is now at a higher pressure than the filter water to start flowing along the connecting pipe into the filter trying to equalise the pressures. As long as the pump continues to pump the water out of the filter, water will flow into the filter from the pond through the connecting pipe. This water is not being pumped into the filter it is moving under its own weight under the influence of gravity. It is moving gently and smoothly. The debris and solids in suspension in the water are not churned up before entering the filter. This makes it easier for the filter to separate out the solids at an early stage which reduces the blocking of the filter media and adds to the efficiency of the filter.
The pipe connecting the pond to the filter will be a very large rigid pipe, typically 4 inches in diameter. This pipe is completely underground, under the pond, below the deepest part of the pond. The pipe is connected to the deepest part of the pond (usually the bottom of the pond will be shaped so that all of the floor slopes down gently towards the pipe entrance. This encourages mud and organic debris to slowly make its way towards the pipe and to be collected into the filter) Where the pipe is joined to the bottom of the pond it is called a bottom drain. The bottom drain will usually be covered by a dome. The dome covers the opening leaving a narrow space underneath for the water to get through. The dome prevents the fish from being sucked into the filter and spreads the suction of the pipe around its circumference so that its pull is less fierce.
The water is pumped out of the final chamber of the filter. This water is then pumped through the ultra violet clarifier and then back into the pond. It is common for this water to be used to supply a stream or waterfall although I would not advise this. It is much better for waterfalls and streams to have their own dedicated pumps. The bacteria in the filter take oxygen from the water. It is a good idea to return the water to the pond in a way that causes it to splash. This helps it to replace the lost oxygen.
Over time mud will accumulate throughout the filter. Excessive mud will reduce the performance of the filter. Blocked brushes and foams will reduce the filters ability to remove further mud and suspended solids from the water. Clogging of the filter media can reduce the water flow through the media and cause a reduction of oxygen and food to bacteria living in parts of the filter. This may cause bacteria in the filter to die reducing the ability of the filter to process the waste produced by the fish. Most filters have valves allowing you to drain down the filter. Either a single valve for the whole filter or individual valves one for each chamber. There will be an isolating valve than must be shut first, before you drain down the filter. This closes the pipe connection between the pond and the filter. Closing this valve means that when you drain down the filter you don't also drain down the pond at the same time.
Foams and brushes should be cleaned once a month. Foams should be squeezed out in a large container holding water taken from the pond. Filter brushes should also be rinsed in pond water and shaken out. Tap water should never be used. It will kill the bacteria living on the filter media reducing the performance of the filter. If the water gets muddy replace it with more clean water from the pond. The filter media should be fairly clean but it doesn't need to be obsessively clean.
The other media will need cleaning too but providing the pond is cleaned out regularly (once a year is good) it will be unnecessary to clean it as often as monthly. This can be done once a year. For best results do it at the same time as the ponds annual clean out.
The bulb contained in the ultra violet clarifier will need replacing every so often. Usually (although this is certainly not true for everyone) the bulb will last at least a year. If the pond goes green change the bulb. Otherwise change the bulb annually or if you prefer wait until the water starts to go green before you change the bulb.
The bulb sits inside a crystal tube. This keeps the water away from direct contact with the the bulb and the electrical connections. The crystal tube allows the light to come through from the bulb into the water where it kills the algae. Lime scale can build up on this crystal tube blocking the light from the bulb and reducing the performance. When this happens the lime scale must be removed, if the ultra violet clarifier it to work at full performance. Soaking the crystal tube in white vinegar will remove the lime scale.
Gravity fed filtration is regarded as the best filtration available to the pond keeper. However, as with most things it does have some drawbacks. The equipment is more expensive to buy and requires greater skill to install. Most ponds are made using pond liners these days. In order to connect the inlet pipe to the pond a hole must be cut in the pond liner and the pipe attached by way of a bottom drain fitting. This is fine if the work is done well and the materials perform as they should. If however in the future the join should fail, it is a very difficult thing to replace or repair.
The connecting pipe from the pond to the filter is in the ground underneath the pond. If this pipe should develop a leak either from imperfect installation or faulty materials, the pipe is completely inaccessible and very difficult to repair or replace without having to first dismantle the entire pond. This is an even greater problem if the pond is made of concrete.
Each of these filter types have their strengths. A pressurised canister filter is a good choice for a small to medium sized pond with a moderate stock of fish. It is discrete and it is clean and easy to maintain. Pump fed filters are suitable for most larger pond sizes. Gravity fed filters should not be chosen lightly. They are really only necessary for people who are aiming to keep lots of very large fish with competing at fish shows in mind.
Do not choose a filter that is only just adequate. Choose something more powerful than you need. Your filter should have spare capacity to give you a good margin of safety. It should be able to cope as your fish increase in size.
Not all ponds are suitable for filtration. Ponds of over 10,000 gallons become increasingly difficult to filter effectively the larger they are. Ponds that have water entering and leaving the pond through say a natural stream or spring cannot easily be filtered as the water is constantly being replaced. Pond filters are designed to deal with a closed system containing a fixed unchanging body of water.
Even the best filter has its limits. Even the best filtration system will not completely protect you against recklessness. What a good filter does is to increase the capacity of your pond. It does not allow you to have an unlimited number of fish. It does not mean you can feed as often or as much as you like. The filter must be well maintained if it is to do its job properly and you must respect its limits. Regular and frequent testing of the pond water is the best way to monitor the performance of your filter and to avoid unexpected disaster.