I have a 55 gallon tank with several various kinds of fish. Recently one of my serpae tetras has a growth/sore where the tail connects to the body. He also has black splotches all over his body. He's more brown then red now. I don't know what disease this is or how to help it. All the tank readings are normal and I do frequent 50% water changes. Any ideas? I don't want it to spread.