It is a 20 gallon tank. PH is at (7.9). My Dalmation Molly is constantly pooping a white (sometimes red) substance. It is littering the bed of the tank. He seems to be too skinny compared to the rest of the fish in the tank. I am fish care ignorant. The people at the fish stores are not very helpful. I was wondering 2 things; First and most important, does anyone know what could be wrong with my fish? Second, can anyone recommend a good book for general fish care and maintenance?