This week I am working on creating a biofilter for a new freshwater tank which will contain a number of mature goldfish. A biofilter provides the necessary conditions for beneficial bacteria to grow. This bacteria is what converts toxic ammonia from the fish into usable nutrients for the plants. This conversion is a two-step process called nitrification where ammonia is first converted by bacteria into nitrites followed by a different species of bacteria converting these nitrites into nitrates. The biofilter itself is a small tank of water containing materials with lots of surface area the beneficial bacteria can thrive on such as lava rocks, plastic pieces, or bunched up netting material. Water flows into the bottom of the tank traveling through the material and flows out of the top where it returns into the fish tank. Because a biofilter depends on living bacteria to function it can take several weeks for a new system to begin working efficiently as the bacteria populations establish themselves.
This information and additional information about aquaponic biofilter systems can be found at http://aquaponics.com/media/docs/articles/Aquaponic-Equipment-The-BioFilter.pdf