Homemade pond filters: a pond bio- filter is essential for clean healthy fish ponds

For good performance, your garden pond needs a bio-filter. The filter, in turn, needs oxygen, a food source, and some sort of biomedia in which the bacteria can grow and thrive.

Don't think you can get away with operating your garden pond without a filter, especially if you keep koi or other fish. But that doesn't mean you have to go out an buy an expensive filter -- in fact, a homemade filter, if constructed correctly, will work just fine.

The easiest pond filter to make yourself is the "black box" type. This type of filter uses a rectangular or other shaped plastic box which contains some or all of the following components:

Open cell foam which is effective in removing leaves and other larger particles from your pond water. This foam is designed to remove larger solid particles from the water before the water enters the biological chamber (a technical term for the place where the media is located). The foam also becomes a holding place for bacteria within the biofilter. This foam has a limited amount of biological capacity.
Brushes, which serve as an alternative to the foam in removing solids and other large waste. Brushes (look like large bottle brushes) – these tend to be found in larger box filters. They are designed to catch solids much like the foam. In very large very well oxygenated filters brushes can make a difference. For the rest they are a waste of money in my opinion.
Plastic tubes, balls, or other plastic shapes which serve as an anchor for biomedia to accumulate and grow. The most common are ridged plastic tubing and things that look like hair curlers. The purpose is very simply to allow surface area to be provided onto which bacteria can create their colonies so that the nitrogen cycle can take place efficiently. These shapes generally have small surface area per unit of volume and they provide little resistance to flow of water through the biofilter.
Inlet and outlet pipes or tubes, and perhaps an overflow pipe back into the pond.

This is just a short list of possible components. In practice, simple pond filters have included almost everything from hair curlers to gravel. The chief benefit of making your own pond bio-filter is to save money.

Constructing a biofilter

You can construct a simple pond filter by first obtaining a suitable container, then adding a solids-catching device such as flat open-cell foam or a set of stiff bristle brushes. This will act as the first stage of your filter. The second stage will be constructed using biomedia, through which pond water flows after leaving the first stage. This pond filter media is a living organism, and it must be kept wet with recirculated water at all times. This is the simplest form of garden pond filter, but if built correctly, it will be just as effective as any store-bought filter.

You need to understand that retail stores will always try to sell their customers large, expensive pond filters, which may not be necessary for all applications. One idea is to have a look inside a commercial filter at the store, study how it's constructed, and then use that as a model for your home-made filter.

A box-type filter operates under gravity, and usually will be placed at the highest point in your pond's water circuit. One important consideration is that water flowing into a gravity biofilter must be able to come out fast enough. You run the risk of pumping you pond dry if your filter can't purge the water fast enough. So always construct your filter with large outlet pipes -- or even two outlets to be extra safe.

Another filter design was the Cyprio Bioforce filter, introduced in 1998. It was the first small-scale pressure filter used in pond keeping. It proved to be a popular design, and since that time many of these pressure filters have been introduced into the marketplace.

Now pleas bear in mind that the above filter has nothing to do with the UV filters you may see advertised. Those types of filters are intended to kill algae and clear the water of its pea-soup appearance, but will not work to remove toxic chemicals from your pond water. For more on this topic, see the pump site, and the article on vegetable filters.

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