I agree with ROKA as to the QT. It is a great possiblity that your fish has "Ammonia burned gills" due to his gasping for air as you stated. A QT is a better option at this point because you can take a clean new batch of SW
or some from the tank and monitor water parameters and do PWC
as needed to keep them in check. It is allot cheaper to do a smaller PWC
in a 10g QT then in a larger DT. Also if medications are needed they will not contaminate the main tank. We all have opinions and they are all with good intentions, but remember they are Opinions. Everyone should step back and see that all we want to do is help correct this situation so that we all can learn from this.