I have a clown knife who developed strange bumps on his face and gills I researched for months without finding any thing but then I heard of a rare aquatic ailment called gas bubble disease it's the fish equivalent of the bends and is caused by over oxidation. If you use water that isn't ruffly the same temperature this can happen apparently this happens alot to arrowana and knife fish. To combat gas bubble disease make sure there aren't any air leaks in your canister filters. I hope this helps!