Fish ‘R’ Fun Bio-Rings Fish ‘R’ Fun Bio-Rings – Replacement Bio Rings for the Fish R Fun range of aquariums. These Bio rings are used in the biological section of the filter. For biological filtration and feature lots of cross section area. This is ideal for bacteria growth that helps steady the quality of the water. Suitable for tropical, coldwater and marine tanks. Rinse in water taken from the aquarium. To achieve maximum efficiency change it when clogged or dirty.