My black fish is sitting at the bottome of the tank and its seems like he has a slight whitish film over him. He also had a bit of growth coming from his mouth. I dont know why its happening. The water is perfectly in balance, the filter is clean and working. He's fed properly etc but still seems to eat only when it falls to the bottom of the tank. I just dont want whatever is happening to him to effect the other fish in the tank. Which is clean and has healthy plants in it. All the other fish seems to be ok.