I would keep him in a small tank on his own with an air stone and filter, dim lights or no light, somewhere to hide, daily water changes (about 20% total volume), offer shelled peas (can help with constipation/swim bladder issues) Use water from the other tank to fill it if possible.

I wouldn't stress about the big tank yet, smaller tanks are better as hospital tanks as the frequent water changes are easier with a smaller volume. Make sure you are using filtered water or dechlirinator Some species are really sensitive to the chlorine and chloramine in tap water.

Bad mans tropical fish website has some great articles on caring for sick fish (it's not just a tropical fish website/forum)

Hope murray picks up soon sorry if im telling you things you slready know xx