If a fish is sick I quarantine it in a small tank with an air stone, treat it if possible and hope it recovers or let it die quietly in peace.
I don't see how that is cruel.
I do the same thing with ill animals. Unless something is so horribly injured that there is no way it will ever recover and is going to be in agonizing pain for an extended period of time. Like that baby turtle I mentioned in a previous post. I have Buddhist leanings and tend to believe that if something isn't suffering badly, then letting it live out its natural lifespan and finish its life cycle is the way to go. I don't think it's cruel. It's just the pain factor that I can't deal with -- I've been around waaaay too many mortally wounded creatures that are suffering and in such severe pain to be able to just walk away and let them continue hurting for any period of time.