Koi in your garden pond need special biofiltration.

The larger the koi in your pond, the larger you filter will need to be.

Ponds that are dedicated to growing large koi are different from other garden ponds. A simple fish pond filter is not adequate for raising large koi.

Koi ponds tend to be deeper than other ponds, and have a higher volume. And large koi eat large amounts of food, and they need more oxygen than other fish. The answer is a highly-efficient koi pond filter with efficient biomedia or koi pond filter media. Japanese matting is ideal for this application.

A typical koi enthusiast will often build ponds that are 10,000 gallons or more and that are home to some very expensive koi. To protect this investment, you will need a very good pond filter system.

A homemade filter designed for a small, 500 gallon pond will not suffice for these larger ponds.

Sand filters not recommended for koi ponds

If you're building a large koi pond, you'll want to stay away from sand filters, even as a so-called "polishing koi pond filter." As a real enthusiast, you should go in the direction of a vortex koi pond filter using japanese matting, or something similar.

Sand filters are just the wrong choice for a koi pond filter system for the following reasons:

• Sand filters are fine for swimming pools, but are expensive to operate as a koi pond filter due to the high-pressure pumps needed to run these filters.
• Sand filters can cause disease problems when used as koi pond filters due to the fact that they cannot be cleaned well enough to prevent pathogenic bacteria from forming in "dead" spots in the filter.
• Sand serves as a very poor filter media for koi ponds.
• Sand filter back-washing systems are not nearly effective enough to clean the filter to the standard required for koi ponds.
• Sand filters waste large amounts of water.
• With a sand filter, it's not possible to add air to your pond. Large koi ponds need massive amounts of air for effective biofiltration, especially after feeding.

In conclusion, sand filters are used because they're inexpensive and easy to install, but they're a bad choice as a koi pond filter.

A vortex filter and Japanese matting water filter

Your best choice for a koi pond filter is a vortex filter and Japanese matting. This has been proven over and over through practical knowledge and worldwide use. This is the favored filter in Japan, where koi raising is an art form. Typically, vortex filters are discussed in the context of use with Japanese matting. Japanese matting is an open-celled material normally supplied in sheet form. It is normally cut into sections that are in turn placed in a lattice pattern at the top of the vortex unit. Then a large amount of air is blown continuously through the filtering system. This rather complex design does not lend itself to the homemade or do it yourself pond filter.

The reason that Japanese matting works so well is because massive amounts of air can be pumped into the voids surrounding the open structure created by the Japanese matting lattice structure. This creates a high specific surface area (SSA) for bacteria colonization. The high oxygen levels creates turbulence, which, couple with the high SSA, makes this type of biofilter perfect for koi pond applications.

This system obeys all the important prinicple of mass transfer, namely:

High concentration differences caused by the turbulence inside the filter vortex chambers mentioned above (Note: this refers to the biological, NOT mechanical chambers).

Large surface area in the vortex chambers of these koi pond filters allows for high mass transfer.

Another advantage of this filter type is that all components, including the much-needed oxygen, come together at an efficient interface.

Now another thing to remember is that some people refer to a UV pond filter, which has an entirely different function than the filters mentioned above. UV type filters used in water gardens are designed to remove the pea soup appearance of the water. UV filters work together with biofilters to both clean the water of harmful chemicals, and make the water crystal clear to look at.

For more information on these subjects, see the pond filters page, and the article on pond filter biomedia.

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