Sorry to hear about your fish troubles. Its never easy...
My opinion, stop dosing anything into the tank. Your tank sounds like its cycling. The fish is gasping from either ammonia/nitrite or a rapid ph drop, old tank syndrome as someone else mentioned. I am sad to say but I think you're making it worse by dosing the peroxide. Fish do not gasp for lack of oxygen but die by an "uneplained", as someone would call it, reasons. Gasping is related to ammonia/nitrite and/or a ph drop. Coudiness doesn't mean the bacteria is kicking in. Cloudy water is due to heterotrophic bacterial blooms, not nitrifying bacteria, which is not good. It is too long to explain but your best chance to save him is fin level daily water changes, dechlorinator and don't mess with the water in any other way. You can just put in a couple of table spoons of salt, but no more. This is to neutralize blood poisoning from possible nitrite.
Best of luck and my best wishes to you and all your family. I am sorry you have to go through all this.