What size pond filter do I need?

Match the filter to the fish, then check it can take the pump's flow. A wildlife pond with no fish usually does not need one.

The RHS split

The RHS filter guide says filters earn their place when a pond has a lot of fish, because they take out waste and debris. Ponds with few or no fish should not need a filter. It separates the work into two jobs:

• Mechanical. Foams and other media catch solids and algae clumps. This works as soon as the water is moving.
• Biological. Bacteria on the media turn toxic ammonia from fish waste into less harmful compounds. The RHS says that population takes about six weeks to build, and it can die if the filter is switched off for 24 hours or more. Do not "give it a rest" for a weekend in summer if fish depend on it.

The same page lists the usual hardware:

• An in-pond filter, often with a pump and UV in one box, for a small, lightly stocked pond. Circulation can be uneven.
• A pressure filter outside the pond. Water goes in under pressure and can be pushed on up to a waterfall. The RHS says these are easier to hide and to clean, and that they should not be used for heavily stocked ponds or large koi.
• A pump-fed box. The pump pushes water in, and the water drains back by gravity, so the outlet has to sit above the pond. You do not run a fountain off that outflow.
• A gravity-fed filter, usually with a bottom drain, for a pond that was built for it. The pump is on the clean side and can feed a waterfall.

If the water is still green after the filter is running, the RHS says the unit may be too small for the fish or the feeding, and that surface area, depth and sun all change what you need. Ask a specialist rather than adding another chemical.

PondXpert's rule of thumb

On PondXpert's sizing page, goldfish can use a pressure filter, and the filter's maximum flow must be at least the pump's maximum flow. Their example pairs a pump of 3,000 L/h with a filter whose maximum flow is higher than that. Koi, they say, need a box filter rather than a pressure filter, again with the filter able to pass at least what the pump sends.

Read the line on the box that matches your fish. A filter "up to 10,000 litres" with no fish, or with a light stock, is not a 10,000 litre koi filter. The planner tells you which line to look for. It does not name a model, because the honest figure is the maker's rating for your stocking.

Where the UV fits

Many boxes include a UV lamp. The RHS describes what that lamp does: it makes suspended algae clump so the mechanical filter can catch them. It is not the biological filter. Lamp size is on the UV page. If the clarifier is a separate unit, it usually sits on the hose between the pump and the filter, which is how Hozelock describes fitting one.

Cleaning without killing the bacteria

Rinse foams in pond water, not under the tap, if you can. Chlorinated tap water is a good way to knock back the bacteria you waited six weeks for. The RHS also says to follow the manufacturer's cleaning interval, and to replace UV lamps on the maker's schedule.

The electricity for the pump is covered on the electrics page. The RHS is explicit that a supply for a filter or pump must be installed by a qualified electrician.

Do I need a filter on a wildlife pond?

The RHS says no, if there are few or no fish. A pump for a waterfall is optional, and it should be guarded so it does not suck in tadpoles. Adding a filter "to keep it clear" often means adding fish later. The wildlife guide explains why that changes the pond.

Why does my filter overflow?

The pump is moving more water than the filter can pass, or the foams are blocked. PondXpert's check is the simple one: the filter's maximum flow should be at least the pump's. Also check that a gravity return is not lower than the pond, or the box will not drain.