Pure Aquarium are unique, clear polymer balls which contain millions of beneficial bacteria and enzymes which help to break down the organic waste that builds up in all aquariums and which can, in some cases, cause cloudy water. Adding Pure once a week (2 balls for every 25 litres) will improve water clarity. Additionally, water quality will also improve, as the bacteria helps boost the performance of filters and helps establish filters in new aquariums. It's ideal for all cold water and tropical freshwater aquariums. To use simply place the required number of balls into your filter, or directly into the aquarium where they will slowly dissolve.