A pond filter helps keep a fish pond healthy by doing two very different jobs. First, it removes solid waste such as fish waste, uneaten food, dead plant material and other debris from the water. Second, it provides a large surface area for beneficial bacteria that process dissolved waste produced by the fish.
A good filtration system therefore does much more than make the water look clear. A pond can look perfectly clean while still containing ammonia or nitrite at levels that are dangerous to fish. For a stocked fish pond, especially a koi pond, filtration should be chosen for the amount of waste the pond is expected to produce rather than for appearance alone.
If you are setting up a pond filter for the first time, the basic principle is simple: remove larger solid waste before the water reaches the finer mechanical media, then provide biological media where beneficial bacteria can establish.
The exact layout depends on the type of filter and whether the system is pump-fed or gravity-fed.
How does a pond filter work?
Most pond filtration systems rely on two main stages: mechanical filtration and biological filtration. Many systems also include a UV clarifier to help control green water.
Mechanical filtration
Mechanical filtration physically removes solid material from the pond water. Depending on the design of the filter, this may be done with filter foam, screens, sieves, brushes, static media, a drum screen or a combination of several methods.
The aim is to remove solid waste before it can accumulate inside the biological section of the filter. This is important because a biological filter works best when water can move freely around the media. If the biological media becomes packed with sludge, both water flow and the amount of oxygen available to the filter bacteria can be reduced.
Where several grades of filter foam are used in the path of the water, larger particles are normally removed before smaller ones. In a conventional foam arrangement this means coarse foam first, followed by progressively finer foam. The finer material is then less likely to become blocked rapidly by leaves, uneaten food and larger pieces of debris.
Biological filtration
Fish continually produce nitrogenous waste, and additional ammonia is produced as organic material such as uneaten food and dead plant matter decomposes. Ammonia is toxic to fish, so a mature biological filter is one of the most important parts of a heavily stocked pond.
Biological filter media provide a large wetted surface on which microorganisms can form a biofilm. Nitrifying bacteria convert ammonia to nitrite and then convert nitrite to nitrate. Ammonia and nitrite are the compounds of greatest immediate concern to fish; nitrate is considerably less toxic but can accumulate and still needs to be managed as part of the overall pond system.
Nitrate may be taken up by pond plants and can also be controlled by appropriate water changes and pond management. A conventional aerobic biological pond filter should not be thought of as a device that simply makes nitrate disappear.
New biological filters need time to mature. A newly installed filter may have plenty of media but very little established bacterial population, so a new pond should not be heavily stocked immediately. Water testing for ammonia and nitrite is much more useful than guessing whether a filter has matured.
For more detail about establishing biological filtration, see how to cycle a fish pond.
What does a UV clarifier do?
Many pond filters include a UV-C clarifier, while other systems use a separate UV unit. As pond water passes through the unit, suspended microscopic algae are exposed to ultraviolet light. This can be very effective for controlling the single-celled algae responsible for green water and improving water clarity.
A UV clarifier is not a substitute for mechanical or biological filtration. It does not remove fish waste from the pond and it does not replace the biofilter that processes ammonia and nitrite. Think of UV as an additional water-clarification stage rather than as the complete filtration system.
UV lamps, quartz sleeves and flow requirements differ between units, so servicing and replacement intervals should always follow the instructions for the particular model rather than a universal timetable.
How to set up a pond filter system
There is no single plumbing layout that applies to every pond filter. The first important distinction is whether the filter is pump-fed or gravity-fed.
Pump-fed pond filter setup
In a pump-fed system, a pump in the pond sends water to the filter. After passing through the filter, the water normally returns to the pond by gravity through a large outlet, waterfall or stream.
A simple pump-fed system therefore follows this general route:
- Water is drawn from the pond by the pump.
- Water is pumped through the hose or pipework to the filter.
- Solid waste is removed by the mechanical filtration stage.
- Water passes through the biological media.
- The filtered water returns to the pond.
This arrangement is common with box filters and many compact pond filtration systems. The filter normally has to be positioned high enough for its outlet to return the water freely to the pond.
Gravity-fed pond filter setup
A true gravity-fed system works differently. Water leaves the pond through bottom drains, side drains or similar large pipework and enters the filter by gravity. The water level inside the gravity-fed filter is therefore related to the water level in the pond.
Mechanical filtration is normally performed before the water reaches the main biological stage. A pump located after the gravity-fed filtration then moves the clean water back to the pond.
This type of arrangement is common on larger koi ponds because large-diameter gravity lines can move solids from the pond without first passing them through a pump impeller.
Basic pond filter setup checklist
- Calculate the pond volume. Do not choose a filter until you have a reasonable estimate of the amount of water in the pond.
- Consider the fish load. A lightly stocked ornamental pond and a heavily stocked koi pond of the same volume do not place the same demand on a filter.
- Check the filter manufacturer's rating. Look specifically for the recommended pond size when fish are present, not only the largest theoretical pond volume shown on the box.
- Match the pump to the filter. The filter will normally have a recommended or maximum flow rate. More flow is not automatically better.
- Allow for real plumbing losses. The flow printed on a pump is not necessarily the flow that will reach the filter after lifting the water and pushing it through hoses, bends, valves and other equipment.
- Make the filter accessible. Any filter that is difficult to open, drain or clean will eventually become a maintenance problem.
- Start the system and check for leaks and restrictions. Make sure the outlet can handle the incoming water and that no chamber can overflow during normal operation.
- Allow the biological section to mature. Do not assume that a new filter is biologically ready simply because water is flowing through it.
What order do pond filter sponges go in?
For a conventional filter in which the water passes through several grades of foam in sequence, the normal principle is:
The coarse material catches larger solids first. Medium and fine foam can then remove progressively smaller particles without becoming blocked as quickly. Dedicated biological media is normally positioned after the main solids have been removed so that it stays relatively clean and receives a steady flow of oxygenated water.
This is a principle rather than a reason to rearrange every commercial filter. Some filters use specially shaped cartridges, differently coloured foams, moving media or a water path that is not obvious from looking at the filter. If the manufacturer specifies a particular media order, use that arrangement.
If you are replacing pond filter foam and do not know which way the media should go, first determine the direction in which the water actually travels through the filter. The coarse side or coarse stage should normally meet the dirty water before the fine polishing stage.
How much filtration does a pond need?
There is no single turnover figure that is correct for every fish pond. Filtration requirements depend on the pond volume, number and size of fish, feeding rate, filter design, amount of biological media, solids-removal efficiency and the real flow of water through the system.
This is particularly important with koi. Koi grow large, eat heavily and can produce a substantial waste load. A filter advertised for a large ornamental pond with no fish may be rated for a much smaller pond once fish are added. Always check the manufacturer's stocked-pond or koi-pond rating where one is provided.
The pump also has to deliver the required flow at the actual head. A pump rated at a certain flow with no restriction may deliver considerably less once it has to lift the water to a filter or waterfall and overcome the resistance of the pipework.
For this reason it is better to size a pond filtration system as a complete system rather than buying a filter according to pond volume and then choosing an unrelated pump solely because its maximum flow figure looks large enough.
Types of pond filters
Modern pond filters come in several designs. The names can overlap because a single product may perform both mechanical and biological filtration, and the same filter may sometimes be installed in more than one configuration.
Box and flow-through filters
A box or flow-through filter is a simple and common design. Water is normally pumped into the filter and passes through mechanical and biological media before returning to the pond by gravity.
These filters are easy to understand and can provide a large media volume for their cost. They also tend to be straightforward to inspect and maintain. Their main disadvantage is that the outlet needs to return by gravity, so the filter may need to sit above pond level and can be more difficult to hide.
Pressurised pond filters
A pressurised filter is a sealed unit that remains under pump pressure. Because the outlet is pressurised rather than being a simple gravity overflow, the filtered water can often be routed onward to a raised return or waterfall within the operating limits of the system.
Pressurised filters are compact and can often be partly buried, which makes them attractive where appearance and space are important. Many include mechanical foam, biological filtration and an integrated UV clarifier.
Their compact size is also a limitation: the amount of mechanical and biological media is finite, and a heavily stocked pond may require a substantially larger or more specialised filtration system.
Gravity-fed filtration systems
Gravity-fed filtration is commonly used on purpose-built koi ponds. Water moves from the pond to the filtration system through large drains without first being pumped. Mechanical filtration removes solids and the water then passes through biological filtration before a pump returns it to the pond.
Gravity-fed installations require more planning than a simple pump-fed box filter because pipe sizes, levels, flow rates and the position of the filtration equipment must all work together.
Moving-bed biological filters
Moving-bed filters use floating biological media that is kept in motion, normally by aeration. The moving media provides a large surface for biofilm while the constant movement helps keep the media exposed to oxygenated water.
A moving bed is primarily a biological stage, so good mechanical filtration before it is valuable. Allowing large quantities of solids to collect in the biological chamber defeats much of the benefit of separating the two jobs.
Drum filters and automatic pre-filters
Drum filters are mechanical filters designed to remove suspended solids through a fine screen. As the screen becomes dirty, an automatic cleaning cycle washes the collected waste away.
They can greatly reduce the amount of solid waste reaching the biological filter and are especially useful on larger or heavily stocked koi systems. The trade-off is greater cost and complexity compared with a simple box or pressure filter.
All-in-one and underwater filters
Small ponds may use compact filters that combine a pump, mechanical media, biological media, UV clarification and sometimes a fountain in one unit. These can be convenient where space is limited, but their capacity should still be matched to the actual pond and fish load.
Which pond filter is best for my pond?
Small ornamental pond
For a small pond with a modest number of fish, a correctly sized compact, box or pressurised filter may provide all of the mechanical and biological filtration required. Ease of maintenance is often more valuable than choosing an unnecessarily complicated system.
Goldfish pond
Goldfish still produce a significant amount of waste, particularly as they grow. Choose a filter with both effective solids removal and adequate biological capacity, and size it for a pond containing fish rather than using an unstocked water-garden rating.
Koi pond
Koi ponds normally justify a more substantial filtration system. Good mechanical waste removal, generous biological capacity, reliable circulation and aeration become increasingly important as the fish grow and feeding levels increase.
For a purpose-built koi pond, it is worth deciding on the filtration system before the pond is constructed. Bottom drains, return lines and gravity-fed filtration are much easier to incorporate during construction than to add later. See also our guide to building a concrete koi fish pond.
Large or lightly stocked natural pond
Very large ponds are a different problem from small ornamental fish ponds. Trying to circulate tens of thousands of gallons through a conventional domestic filter at a high turnover rate may be impractical and expensive. The correct solution depends on fish density, water source, depth, aeration, planting, nutrient load and the goals for the pond.
A large, lightly stocked pond may rely much more on natural processes than a small koi pond. This is one reason that simple rules such as choosing a pump from pond volume alone should not be applied blindly to every body of water.
How to clean pond filter media
Mechanical filtration only works if the trapped waste is actually removed from the system. Foam, screens, baskets and other mechanical stages should therefore be checked and cleaned often enough to prevent them from becoming badly restricted.
The correct cleaning method depends on the filter. Some modern filters have built-in cleaning systems or media that is agitated and flushed without being removed. Others use removable foam or cartridges. Follow the servicing procedure specified for the particular filter rather than assuming that every type of media should be cleaned in the same way.
Biological media needs a different approach. The aim is not to sterilise it. A mature biofilm is exactly what makes the biological filter useful. Avoid unnecessary disinfecting or aggressive cleaning of established biological media, and do not replace all mature biological media at the same time unless there is a specific reason to do so.
Where foam performs both mechanical and biological duties, gentle cleaning that removes accumulated sludge while preserving as much of the established biofilm as practical is preferable to treating the media as though it needs to be made sterile.
Should a pond filter run all the time?
During the active pond season, a biological filtration system is normally designed to run continuously. The bacteria in the biofilter depend on a regular supply of oxygenated water carrying the waste compounds that they process. Repeatedly switching the filter off for long periods, such as every night to save electricity, is therefore not a good operating strategy for a normally stocked fish pond.
Winter operation is more dependent on climate, pond design and equipment. Some systems continue to operate throughout winter while others are partially or fully winterised. Follow the manufacturer's frost-protection instructions and make sure any change in circulation does not compromise fish welfare or allow equipment to freeze.
Can you build your own pond filter?
Yes. A simple pond filter can be built from a suitable container with separate mechanical and biological stages, and home-made filters have been used successfully for many years.
However, a DIY filter still has to meet the same basic requirements as a commercial one. It needs enough flow and media capacity for the pond, a safe way to remove accumulated waste, an outlet that cannot be overwhelmed by the inlet flow, and easy access for maintenance.
Any container used as part of the water system should be made from material suitable for the purpose and should be structurally capable of supporting the water it contains. Pipe penetrations need to be secure and watertight, and an overflow or blocked outlet must not be able to empty the pond or flood nearby electrical equipment.
Pumps, UV units and other electrical equipment used around a pond must be designed for outdoor aquatic use and installed in accordance with the electrical safety requirements that apply in your location.
Common pond filter mistakes
- Choosing a filter by pond volume alone. Fish load can make a very large difference to the amount of filtration required.
- Using the largest pump possible. Every filter has a useful operating flow range. Excessive flow can be just as problematic as inadequate flow.
- Ignoring head loss. The flow printed on the pump is not necessarily the flow that reaches the filter in the completed installation.
- Putting fine mechanical media before coarse media. Fine foam can clog rapidly if it has to collect all of the largest debris as well.
- Allowing the biological section to fill with sludge. Good mechanical filtration should remove as much solid waste as practical before biological treatment.
- Cleaning biological media too aggressively. The established biofilm is an important part of the filter, not dirt that needs to be sterilised away.
- Stocking a new pond too quickly. Water flowing through new filter media does not mean the biological filter is mature.
- Assuming clear water means safe water. Ammonia and nitrite cannot be judged by looking at the pond. Use appropriate pond water testing and water-quality checks.
- Making the filter difficult to service. Leave enough room to drain, open and clean every part that requires regular maintenance.
Fish pond filter FAQs
What order do pond filter sponges go in?
In a conventional multi-stage foam filter, arrange the mechanical media from coarse to fine in the direction of water flow. Larger waste is removed first, which helps prevent the fine foam from clogging too quickly. If your filter manufacturer specifies a different arrangement, follow the manufacturer's instructions.
How often should pond filter sponges be cleaned?
There is no useful fixed interval for every pond. Clean mechanical media when the accumulation of waste begins to restrict flow or when the filter's maintenance indicator or instructions call for cleaning. A heavily stocked pond during warm weather may require much more frequent maintenance than a lightly stocked pond.
Does a pond filter need a UV clarifier?
Not necessarily. A UV clarifier is particularly useful when suspended single-celled algae are causing persistent green water. It can improve water clarity, but it does not replace mechanical or biological filtration.
How long does a new pond filter take to mature?
Biological filtration does not become fully established immediately. New filters commonly need several weeks for a substantial nitrifying population to develop, and the exact time varies with temperature, water chemistry, oxygen availability and waste load. Test ammonia and nitrite rather than relying only on the calendar.
Can a pond have too much filtration?
Having more biological media capacity than the fish currently require is generally less of a problem than having too little, but oversizing every part of the system does not automatically improve the pond. Excessive pump flow, poor plumbing design and unnecessarily high energy use can create new problems. The pump, filter and pipework should work together as a system.
Should the pond pump go before or after the filter?
Both arrangements are used. In a pump-fed system the pump is before the filter and sends pond water to it. In a true gravity-fed system, water reaches the filter by gravity and the return pump is positioned after the gravity-fed filtration stages.
Does every pond need a filter?
No. A fish-free or very lightly stocked wildlife pond can often function as a largely natural ecosystem without powered filtration. Once substantial numbers of fish are added, especially koi or heavily fed goldfish, the amount of waste increases and mechanical and biological filtration becomes far more important.
Reader question
I have a pond,dug into clay, @30' X 80' X 4' (avg. water depth), with a trench originally dug (Sept. 2008) another 7' deep X 4' X 15', which I know has partially caved in. I've estimated water volume @62,746 U.S. gallons. I have an aerator which stirs the water very well, which I use a few times a week. The only plants are Cattails. I have had fish (Koi, goldfish) and frogs living in the pond since we built it, but a lot of them died this winter I think from ice covering the pond for 4 months (lack of oxygen) which is not normal (I live in S.W. Ontario, Canada). Before I restock I'd like to install a gravity flow filter. Your article on filters, while informative, raised questions. I am not going to filter pond twice an hour due to volume and cost of electricity. More likely a few times a week, at night when elect.costs are lower. Will this still give me the proper results, or will I be wasting time and money? I have had lots of algae the last couple of springs (2012,2013) which I have raked up from the edge of pond. Any advice will be welcomed
As you have rightly stated running a filter for that size of pond would involve an enormous set up and huge amounts of power to run it. You would probably be best adding lots of oxygenating plants and continue with the aerator as you already are. To help with the algal problem in the springtime it is possible to add the barley straw bales to combat this problem.