I dont think there's much you're going to be able to do to keep them long term. Those fish get far too large for such a small tank. Cramming them in is only going to cause aggression issues.
As far as treating the fish in your case I would let the healthy fish go while keeping the unhealthy one alone so it can recover before releasing that as well.
"The simplest explanation for some phenomenon is more likely to be accurate than more complicated explanations." -Occam's razor