I have 2 chromis and a wrass in quarantine right now. They have been there for just over a week. All are eating anything I have put in the tank. I have a large airstone also going in the tank. I noticed tonight that my one chromis was swimming tonight at the surface with his head straight up and he was acting like he was sucking for air at the surface. There is no ammonia in the water. The other fish are acting perfectly normal. It is a 20 gallon tank, so I did a 5 gallon water change on the tank to get some fresh water in there. Any other ideas? Should I do a larger water change?