Bio balls are used as a filter medium in trickle and canister filters. They are designed to promote good water flow and increased surface area. The material used in construction is inert plastic. The channels and grooves inside the balls are colonised by helpful bacteria. These microorganisms biologically filter the aquarium water by changing ammonia into nitrite and finally nitrate.
Bio balls can be fairly expensive compared to other filter media but unlike many other filter media solutions bio balls are not thrown away after use. Instead they can be washed and replaced back into the filter. Bio balls will be useful for years after being purchased.
Aquarium Filter Reading