Hydor Bio-Rings for freshwater and saltwater aquariums are created through the high-temperature sintering of inert quartz. Thanks to the macroporous structure, Hydor Bio-Rings is the perfect substrate for biological filters where the main goal of the hobbyist is the development and growth of bacteria colonies (aerobic and anaerobic) on the large surface area available (8.25 m2 per litre of Bio-Rings). Bio-Rings do not release phosphates, heavy metals, and does not change the pH of the aquarium.