What It Does

• A biological filter houses bacteria that convert ammonia (animal waste) into nitrite and then nitrite into nitrate, which is absorbed by plants as food. If these bacteria were not present, life in the pond would die.
  
  

Size

• The size of a biological filter is determined by the size of your pond. The bio-filter will, over time, filtrate all of the water in the pond. Its capacity should be 10 percent of your pond's total volume. For a small pond, the size will be minimal. For larger ponds, a series of tanks may be needed to adequately filter the water.
  
  

Pump

• Start by placing a pump in your pond, preferably at the bottom so that it more easily reaches all of the water in the pond. Select a pump that will control water flow at one to two gallons per minute, or 60 to 120 gallons per hour.
  
  

Cleaning the Water

• Connect the pump using a pipe to a container set up to clean the water of debris and sediment. Brushes and foam pads can be placed in the container to act as cleaners. Place a piece of mesh or netting in front of these to prevent fish, other animals and plants from entering the filter.
  
  

Bio-Filter Media

• The bio-filter media are what the bacteria will attach and grow on that you need to purify your pond's water. Plenty of surface area is key here. Examples of excellent choices of media are lava rock, volcanic rock, plastic bio balls, bio ribbons, or do-it-yourself media such as cut up pieces of plastic pipe, or old film containers you haven't found a use for. Place your bio-filter media in a plastic mesh bag for easy removal when you need to clean off any collected debris or sediment.
  
  

Small Pond Bio-Filter

• A small pond can have a bio-filter in one container. One example is to have the water enter the top of the container, pass down through brushes and foam pads to clean it, then over the bio-filter media at the bottom before it passes out of the container back into the pond. If your pond has a waterfall, use a smaller container for the brushes and foam and place the bio-filter media in the top of the waterfall.
  
  

Large Pond Bio-Filter

• A larger pond will need a larger filter system to handle all of the water. Two or more containers may be needed for removing debris and sediment, then two or more containers may be needed for the bio-filter media. Arrange the containers so water is able to flow top to bottom.