Biological Filtration should be the backbone of any filter system. Biological filters take advantage of the Nitrogen Cycle. The Nitrogen Cycle starts when your fish excrete waste or when any uneaten food starts to decay. The fish waste and/or decaying food releases toxic Ammonia into the water. The bacteria, Nitrosomonas, lives of of Ammonia and will convert it to the less toxic, Nitrite. Then, the bacteria that lives off of Nitrite, Nitrobacter, converts the Nitrite into harmless Nitrate. Which is food for any aquatic plants.
A good biological filter will clean your water efficiently, be easy to clean, and will have adequate surface area for the bacteria to grow on. Biological filters range from simple boxes filled with rock to state-of-the-art pressurized bead filters.