I think 20 gallons is pretty small for such a big fish. It would stress the fish more than anything, plus dats get easily stressed. IMO, just leave the fish in the big tank, it looks happy there.
I cannot say anything abot the other med, but Melafix has work well for me in the past. Just be careful with the dosing, always read the label. There is two Melafix out there, one "normal" one and the other is for pond, which is more concentrated.