Bio Balls are used for biological filtration which is the action of bacteria in the tank breaking down dangerous ammonia. The ammonia is converted to nitrites and then to the less toxic nitrates. They are a lighter and more effective solution to biological filtration than traditional porous stone.
Bio balls sit in your filter. As water passes over these balls, the nitrifying bacteria filters your water, removing ammonia and nitrites – dramatically improving the water quality.
The main advantage of bio balls over other biological media is that the surfaces are impossible to clog up (when used with a pre-filter). The reason is simple: the gabs of space within a bio ball is large and anything that can clog it up would already have been filtered by the pre-filter (such as filter wool, sponge etc). This is the reason why water filtration should always pass through mechanical filtration (pre-filter) before biological filtration.
How many bio balls do you need?
A general rule of thumb is 45 litres of water per litre of bio balls. While adding fewer bio balls than recommend results in inadequate filtration, there is no harm in adding more.