High quality Activated filter Carbon pellets.
Activated Carbon serves two jobs in the aquarium, firstly it is highly absorbent removing any cloudiness, impurities, colouring and chemical build ups within the aquarium. The carbon will, unfortunately, absorb any chemical used to treat the aquarium, so it is important that it is removed if any treatments are being used.
The second job of the carbon is a fine polishing this is done after the filter has removed larger debris, carbon will trap and absorb fine particles.