Can I ask why not to use Bettafix? Not because I don't trust your opinion, just because I am interested. :)
I have not been able to notice any bloating, that was the first thing I looked for. His stomach looks a bit darker than normal but it seems to have just been because of his color loss.
I wouldn't say he is gasping for air at the top, but any time he does go up (sometimes he seems to be triggered by me being around the tank/taking the lid off) he just seems very quick to the top, takes a quick breath and goes back to the bottom. He is not at the surface very often, which is also very unusual for him.
He has not been rubbing against anything and I haven't noticed any kind of bumps, spots, or dust on his body. He has had a very light and shiny blue spot on one side since i got him, has not grown or changed since so I figured it was just an odd pigmentation. I will post a picture of him at a healthy stage where you can see what I mean.
I bought some aquarium salt today, the box said to add 1 tablespoon per 5 gallons of water which worked out to just over half a teaspoon for my tank (3ml). I know you had all recommended more but I wasn't sure if there was a brand specific limit to it at all so thought I would double check with you, should I add more?
I did see stress coat while i was out for the salt, and would have grabbed some had I known bettafix was not ideal before now!
I do acclimate him well and slowly when i do the water changes, I just feel like scooping him out of his tank every day must not be pleasant for him (maybe I'm just over sensitive?)
I considered ammonia poisoning as well but I haven't noticed any sort of inflammation around his gills or anywhere else. I guess getting my water tested will be my next step if the salt and everything doesn't work.
P.S. after looking at some old pictures of him I am quite convinced of fin rot though he doesn't come out now enough for me to see much of his fins, they were gorgeous back then.. not sure whether it's fin rot or just him looking sick in general...
And this is all I see of him now at best... :(