I have a 55 gal community tank. Things were going well until recently I added 2 discus fish and now on one side of the tank there is a tremendous growth of "black spots" on the gravel rocks, the artificial plants and the stones. There is none on the fish; they appear healthy, nothing unusual. On both sides of the tank there is equal lighting but the fungi growth appears heavily on one side and just a little on the other. What's wrong with my tank? The fish seem healthy in fact they're breeding.
