I'm new, please forgive me if gravel fungus doesn't make sense :) A friend of mine has a 55 gallon tall freshwater tank. It was established about about a year ago. She has gotten some kind of cottony fungus on the gravel. We broke down the tank removing everying but the gravel, cleaned it and put it back. The stuff came back. It's not on the fish only the grave. She has a big canister filter on the tank as well as a UV sterilizer. I've never seen this in any of my tanks. Any ideas? thanks much.