However, a filter need not be quite as visible as our picture shows but can be incorporated into a waterfall or cascade system quite easily; hiding the filter box in a pondside rockery is common practice. 
However, a filter need not be quite as visible as our picture shows but can be incorporated into a waterfall or cascade system quite easily; hiding the filter box in a pondside rockery is common practice.
If a fountain is installed, do appreciate that the pump-protecting sponge filter on its input is NOT a pond filter in any way - don’t depend on it to clean the pond!
Choose a pump that circulates half the pond capacity per hour; remembering to allow enough extra pump power to raise the water up to the filter box or waterfall head, often at least three or four feet above the surface of the water in the pond.
External filter units are fed from a submerged water pump although some filtration units can be used in the pond itself fitted to the input side of the submersible pump.
Different filter materials do different jobs: some (known as mechanical media) strain debris from the water whilst others (known as biological media) provide a home for bacteria colonies which convert toxic ammonia-based compounds into less harmful nitrates. It is also possible to use chemical media which may remove unwanted substances from the water by adsorption or resin exchange.
Maintaining the filter should be a regular task. Rinse out filter medium (sponge materials) using some pond water so as not to destroy, or retard, the nitrifying bacteria colonies living within the media..
As mentioned earlier, Koi ponds lack water-purifying plants and must have a filtration system fitted to keep the water conditions at their best.
Koi are heavy feeders and produce a lot of waste, so the filtration system needs to be big enough to cope.
Filter units (at least as large as the one shown here) are sunk into the ground alongside the main pond and covered with decorative decking. The water enters the filter through a side port in the pond wall or by a bottom drain and associated pipework before being pumped back into the pond by an 'above surface' pump.
The filter shown is a 'vortex' type where water enters at a tangent and is whirled round with the dirt falling down the centre to the bottom from where it can be drained off.
Because oxygen is used up during the biological process, filtered water is re-oxygenated by means of a venturi device on its way back to the pond.
What Kind of Pond?
Choosing a Liner
Installation Tips
Fish & Plants
Dealing with Green Water Problems
Useful Facts & Tips
Last updated January 17, 2002