Activated carbon has been used for years and is very popular to use in both freshwater and marine aquaria due to its effect at making aquarium water look sparkling clear. The effect of activated carbon on marine fish in aquaria is well known. Heavy use of activated carbon over prolonged periods of time can lead to the development of hole in the head disease, also called lateral line erosion in fishes. This effect is commonly seen in tangs and angelfish, but can even be seen in damselfish when carbon is used continuously over a long period of time. However, the effect of activated carbon on corals is not well documented in reef aquaria. It is our experience that the heavy use of activated carbon in reef aquarium can kill corals. This effect will show when otherwise healthy corals suddenly start bleaching from the tips.