Contrary to common belief, biological filters do not process or filter the solid waste of fish in your koi pond. They continue to build up and putrefy, creating a breeding ground for harmful species of heterotrophic bacteria which are pathogenic to koi fish. As stated, the biological filtration process utilizes Nitrosomonas bacteria to break down ammonia into nitrite and nitrobacter, further converting nitrites into nitrates, which is less harmful to koi fish.
Plants now utilize the nitrate and phosphate for fertilizer; if you have not provided an adequate ratio of water plants to koi fish, "hard" algae (growing on rocks and koi pond walls) and "free- floating" algae use nitrate and phosphate to reproduce.
The key to preventing this condition, called "algae bloom," is to provide enough non-soil bearing plants such as water lettuce and hyacinths to compete for the nitrate and phosphate. Since these two plants are tropical and can only survive in warm climates, hardier varieties such as Elodea and Anacharis will perform well in cold climates. These plants are commonly used in bio-filter ponds.
A bio-filter pond is used in conjunction with shallow koi ponds with small populations of koi fish. It will also eliminate the need for a mechanical bio-filter and a second pump to operate it.
The bio-filter pond is located higher than the main pond for two reasons. It prevents the koi fish from eating the plants and it allows the water from the waterfall to be filtered as it passes through the plants prior to spilling into the lower koi pond.
An adequate ratio of plant cover for the koi pond's surface is approximately 20 to 30 percent. This is a basic rule of thumb and many factors can change this equation.
For example: koi fish population, water temperature, and debris accumulating from leaves or over-feeding the koi fish. I have said many times that the Koi Pond is the koi's living room, dining room and toilet.
If you do not have a bio-filter, that could explain why your koi pond is a tad green, stinky, or cloudy, and why your finned family is gulping air on the surface. Trust me. That will not be for long. Fish gulping air to survive would be like you -- in an attempt to avoid breathing poisoned air -- gulping water to survive.
If you do not already have a biological filter, I can not encougage you enough to abtain one. If your pond is over eight hundred gallons, and you are using an above ground pump that produces over 3500 gallons per hour, I suggest a pressurized bead filter. I cover the topic of biofilters in greater detail in another article. Happy koi, peace and joy.
