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Biological Filters

When it comes to Biological filtration, you can never have too much filtration.

Some state of the art Koi ponds have biological filters that are 10 to 20 percent the size of the pond. Even though adequate filtration is necessary for healthy Koi, impurities in the water build up over time; frequent water changes (10 to 15 percent per week) are helpful in eliminating impure water.

Unlike a swimming pool filter, which is to keep the water clear and clean, it is the chlorine added to the swimming pool which keeps the algae from growing on the sides of the pool and is really responsible for the clear water. In a Koi pond, we cannot use chlorine and we want the fine velvet coating of algae to grow on the sides of the pond.


In fact, if algae does not grow on the pond sides then there is a major water quality problem. Your Koi nibble on this algae, providing nutrients, giving them their luster and color.

We also want the water to be clear so that we can always see our Koi. Therefore something other than chlorine must be used to keep out the unicellular algae (causes pea-soup water) and the long stringy algae. These are the purposes of the Biological Filter.

Likewise, just because the water is clear does not mean that it is healthy for the Koi. In fact, the ideal water is clear with a slight greenish tint, as well as being odorless and tasteless.

A filter must be broken in gently over the first few months of its life. A vital part of filtration is living, unlike mechanical filtration, the bacterial colony takes time to become established. The steady turnover of water through the filter provides the bacteria with a constant source of “food” in the form of ammonia, as well as an essential supply of dissolved oxygen. Add a few fish at a time, so that the bacteria can adjust and catch up with the rate of ammonia being produced by the fish.

Power outages for 3-4 hours can deplete the oxygen in your filter and allow anaerobic bacteria to begin growing, aerobic bacteria (good bacteria) begin to die off.

Seeding the Biological Filter

If the pond or filter is new or if the biological filtering system has crashed one must basically start from scratch to seed his filter. Below are listed ways to kick start the growth of nitrifying bacteria.

Make Sure the Biological Filter is Big Enough — One can never have too much filtration. The heavier the fish load the more filtration is needed. If the filter is not adequate it will need constant cleaning and will not do a good job of housing enough bacteria.

Add Enzymes — Some enzyme products are better at helping build up the bacteria than others. MicrobeLift PL is our favorite for encouraging the growth of nitrifying bacteria while MicrobeLifts Super Start Bead Filter Bacteria is wonderful for systems with bead filters.

Hold Off Cleaning the Filter — As long as the filter is not clogging up the water flow and until the nitrifying cycle is established do not clean the filter. If it must be cleaned use de-chlorinated water from the pond.

Turn Off the UV — While the filter is seeding unplug the UV. Green water may be ugly to some of us but it is not necessarily unhealthy. It is better to allow the nitrifying bacteria to establish well before turning on the sterilizer.

Do Not Feed the Fish — The more food the fish eat the more waste in the system to convert to fertilizer. Resume feeding when the tests for ammonia and nitrites come back negative for a week

Perform Water Changes — Test the water every day. Change the water (remember to de-chlorinate tap water) if the ammonia is over 1.2 ppm and/or the nitrite reading is over 2 ppm if using Ultimate. If not, change the water whenever there is any reading of ammonia or nitrite. Keep monitoring and making water changes as needed until the filter is seeded.

Thin Out an Overcrowded Pond — It is hard to make the decision of who stays and who goes but it’s for the good of all the fish. Give the extras away or build another pond for them.

Mechanical Filter

Mechanical filters remove suspended debris or particulate matter from the pond. Most filtration systems remove surface debris through the use of skimmers or a settling chamber located at the start of the filtration system. As its name implies, a skimmer removes surface debris from the top layer of the pond. A settling chamber is an empty tank, usually situated before the biological filtration unit, that removes suspended matter before it is passed on to the biological filtration tanks.

For information on Bog Filters Click Here

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