Terrible trouble with fishtank. Please help!?

About 5 days ago, I noticed my 2 goldfish were lethargic and mostly laying on the bottom of the tank. The water is perfect (ph levels, nitrates, nitrites, etc), temperature is good, but they're just not acting like themselves. I had added 4 serpe tetras to the tank about a week before that. I panicked and took the serpe tetras back to the fish store and picked up some Pimafix, which I've used for 3 days now. Now tonight, I've noticed that ALL the fish are not acting right. The 3 remaining serpe tetras are kind of just floating around aimlessly, my catfish is coming out of hiding which is very unusual for him and my little algae eaters are pretty lethargic too. Does Pimafix affect the tank this way or is there some m infection of something going on here? I'm really upset. I'm thinking about doing a 50% water change to try and help them.