Bag of Bio Balls
Bio Balls are used for biological filtration. 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.