A good filter keeps pond water clear and your fish healthy. Here's how pond filters work, which type suits your pond, how to size one and how to keep it working without killing the helpful bacteria inside.
By Pond.com editorsUpdated 6 min read
Quick answer
A pond filter does two jobs. It traps debris, and it hosts bacteria that turn toxic fish waste into safer compounds.
Choose a filter rated for at least your pond's volume, and about twice your volume if you keep koi or lots of fish.
Make sure your pump's flow at head height falls within the filter's recommended flow range.
A new filter takes around 4 to 6 weeks to build up its bacteria. Add fish gradually.
Rinse filter media in pond water, never tap water, and keep the filter running 24 hours a day.
A pond filter is what turns a hole full of water into a healthy home for fish. Without one, fish waste and rotting leaves build up, the water turns cloudy and toxic, and algae takes over. With the right filter, sized properly and looked after, the water stays clear with surprisingly little work.
This guide explains how pond filters work, which type suits your pond, how to size one and how to clean it without undoing its good work.
How pond filters work
Pond filtration does two different jobs, and most good filters do both.
Mechanical filtration catches solid material such as leaves, fish waste and sludge. Water passes through brushes, sponges or mats that trap the debris before it can rot.
Biological filtration deals with what you can't see. Fish produce ammonia, which is toxic even at low levels. Helpful bacteria living on the filter media turn ammonia into nitrite, and then into nitrate, which is far less harmful and which plants use as food.
Biological filtration is the more important of the two for fish health. Clear water can still contain enough ammonia to make fish sick, while slightly cloudy water can be perfectly safe.
There's a third piece that's often sold alongside filters: the UV clarifier. It doesn't filter anything itself, but it kills the free-floating algae that turn water green, so the filter can catch them. See our guide to pond UV filters.
Catches leaves before they sink; often houses the pump
Waterfall filter
At the top of a waterfall
Ponds with a waterfall
Biological filter that doubles as the waterfall's source
Pressurized filter
Beside the pond, can be buried
Small and medium ponds
Pumps water uphill; often has UV built in; easy backwash cleaning
Gravity (box) filter
Beside the pond, above the water level
Medium and large fish ponds
Large capacity; water flows back by gravity, so the box must be higher than the pond
Submersible filter
On the pond floor
Small ponds and water gardens
Cheap and hidden, but you have to lift it out to clean it
Bead or drum filter
Beside the pond
Large koi ponds
Very effective but expensive; often self-cleaning
Many ponds combine two types. A skimmer plus a waterfall filter is a popular pairing: the skimmer catches debris at one end, the pump inside it sends water to the waterfall filter at the other end, and the whole pond circulates from end to end.
How to size a pond filter
Filters are sold by the pond volume they can handle. Those ratings usually assume a lightly stocked pond in mild weather, so build in some margin:
Plants or a few small fish: a filter rated for at least your pond's volume.
Goldfish or a well-stocked pond: around 1.5 times your volume.
Koi: around twice your volume. Koi are big, hungry fish and produce a lot of waste.
Sunny ponds also need more filtration than shady ones, because more sunlight means more algae growth.
If you're not sure of your pond's volume, multiply length × width × average depth in feet, then multiply by 7.48 for gallons.
Every filter has a recommended flow range in gallons per hour (GPH). Too little flow and the filter can't keep up. Too much and water rushes through too fast for the media to work, or overflows the filter box.
Check your pump's flow at your actual head height, not the maximum figure on the box, and make sure it falls inside the filter's range. Our pond pump guide explains how to work this out.
A new biological filter is clean but empty. The bacteria that make it work need time to grow, usually around 4 to 6 weeks once the water is above about 50°F. This is often called "cycling" the filter.
Start the filter before you add fish, or add just a few fish at first and build up slowly.
Use a bacteria starter from a pond store to speed things up.
Test the water. A simple pond test kit shows ammonia and nitrite. When both read zero, the filter is ready for more fish.
Feed lightly while the filter is getting established, as uneaten food adds to the waste.
How to clean a pond filter
The aim is to clear trapped debris while leaving most of the bacteria in place.
Switch off and unplug the pump before you open the filter.
Fill a bucket with pond water. Tap water usually contains chlorine or chloramine, which kills the bacteria.
Rinse the sponges, brushes or mats in the bucket until they're no longer clogged. They don't need to look new. A brownish film is the bacteria you want to keep.
Don't clean everything at once. If the filter has several sponges or trays, clean half now and half in a week or two.
Check the parts as you go. Look for cracked housings, worn seals and blocked pipes, and replace like for like.
Restart straight away. The bacteria start to suffer within hours without fresh, oxygenated water.
Pressurized filters usually have a backwash setting, which flushes out debris without opening the filter. Run it until the water coming out looks clear.
How often?
Clean the filter when the flow drops noticeably or the filter starts to overflow. In summer that's often every few weeks. In spring and fall, especially under trees, you may need to empty the skimmer basket every day or two.
Filters in winter
Bacteria slow down sharply when the water drops below about 50°F, and fish eat much less, so the filter has less to do.
In mild climates, keep everything running all year.
Where the pond freezes, many owners shut down external and pressurized filters in late fall. Drain them, clean them and store them somewhere frost-free so trapped water can't freeze and crack them. Keep a small pump or pond de-icer running to hold a hole in the ice for gas exchange.
In spring, restart the filter once the water warms up, and add a bacteria starter to get it going again.
Common questions
If you keep fish, yes. Fish waste produces ammonia, which builds up quickly in a pond without filtration. A plant-only pond can stay clear with good planting and a little circulation, but a filter makes it much easier.
Start with your pond's volume in gallons. Choose a filter rated for at least that, and roughly twice that for koi or heavily stocked ponds. Maker ratings usually assume a lightly stocked pond in mild weather.
Clean it when the flow drops noticeably, often every few weeks in summer. Over-cleaning does more harm than good, because it washes away the bacteria that keep the water safe for fish.
No. The bacteria in the filter need a steady flow of oxygenated water, and they start to die within hours without it. Keep the pump and filter running around the clock.
Green water is caused by tiny free-floating algae that pass straight through most filters. A UV clarifier fixes this by clumping the algae so the filter can catch them.
