Evolution Aqua PURE AQUARIUM are biodegradable gel balls crammed full of live bacteria and enzymes for adding to your power filter. They get to work breaking down ammonia and nitrite whilst cleaning up organic waste. Can be used when setting up a new aquarium and for routine maintenance. Keep these in your filter to maintain excellent water quality. Available in 6, 25, and 50 ball packs. 6 balls treats up to 120L.