What is the water temp, and what are you feeding him?
Do you have and air stone in the tank?
What is the GH/KH?
When you first introduce Discus into a new tank there is at least a two week acclamation period. Sometimes they will even lay on the bottom of the tank as if they were sleeping or worse. As with angel fish there is always going to be a pecking order. If you stick three angel fish in a tank there is going to be a large one, medium one, and a small one, same goes with discus, it’s just the way they are set up.
I have two discus in a 55G and one is much smaller than the other and his stress bars are always going except right before I turn the lights on in the morning. He does eat, but he doesn't like to come to the top with the other guy, so what I do is either drop in two blocks of BH and float a couple blood worms on the top, that way they have a choice of what to eat, so they aren't as bothered as much. I also introduced granules that slowly float to the bottom. I think they like this method to eat as well because they can eat all day long at their own pace. I also stuck a long air stone in the middle of the tank to break up each side, that way if they get sick of each other they have somewhere to hide and get away from the other one. Now they are always paired up, I never see them without each other. The little guy is getting bigger, but still not as big as the other one.
I would call your LFS and tell them what is going on so you can get your money back if he does pass, but highly doubt that one will just not eat until he starves unless he/she is already unhealthy if you switch up the eating methods. Give it time, it will work its way out.
). I had 2 discus a while ago and the same thing happened. One ended up dying. You need to get another Discus. I actually think that getting two would be better. Or get rid of the Discus. Aslo do more research on the best tank mates, as some of yours are not the best.
