I've had my oscar for about a month now. He is only 3-4 inches long. A couple of weeks ago he was beaten up by some convict cichlids that i've since gotten rid of, so he is now the only fish in the tank, which is 20g (I know it's too small - I'm going to get another one eventually). I treated him with stress coat and salt for a few days and he recovered nicely. We were getting on just great and he would come up for me to feed him, but for the past week he has not been eating and has been laying on the bottom of the tank, sort of tilted to one side. He swims backwards a lot too. I haven't checked the water but I did a 50% water change two days ago and added stress coat, and it doesn't seem to have helped. Could this be an internal parasite? Should I treat him with some sort of antibiotic or should I wait to see if he recovers? He has no external injuries or other signs of stress. Thanks!

When you say you have checked the water, does that mean you have done the ammonia, nitrite, nitrates and pH? If so, what were the readings for all of them?

Does the Oscar look bloated in any way? Is he breathing heavily? In other words, opening and closing his mouth a lot. When you do a water change, how do you do it? Do you clean the filters at all? An Oscar that stops eating for quite a while isn't a good sign I am afraid. Get back to us with some answers and we will try and help. I must warn you that we may not be able to give you a conclusive diagnosis and cure.