Don't know how high the tank is but you can get a gallon size water bottle (Crystal Geyser's are clear so you can see the fish although they have those speed bumps on them) and use it as a hospital tank for now. You can pour out part of the gallon to give more surface space, put meds in and shake up. Let the gallon float in the big tank until the water warms up and put the fish in, secure the water bottle to the top/side of the tank so it doesn't float away or submerge. You can probably leave the top off so air can get in--but you might consider slipping a nylon stocking or netting over the top in case the fish tries to jump out. At least he'll have nearly 1 gal. to swim around in (albeit it's vertical space), you can keep an eye on him yet keep him warm in the big tank, he can still see his roommates. It shouldn't take too long for his fin to heal but treating a whole 29 gal. tank for just one fish is crazy and some fish do not react well to Melafix (I stopped using it since too many of my bettas got horrible sick with Melafix).
