When we have sick critters, for many of us we reach a crossroads where we have to make the decision: to live or not. It's so very hard, but we have to be unselfish and figure out if the quality of life is gone, beyond return, and euthanize before more suffering occurs. To allow a creature we've made dependent on us live on in agony is a horrible thing to do, yet so many do it to their dogs, cats, etc. I had to make the decision to put down my 2.5 month old filly 9 years ago; she would have been a cripple, and eventually collapse, the joint gone, by the time she reached mature weight. It almost destroyed me, yet I still know I made the right decision. IMO, a fish is just about as much a companion, and it's quality of life taken just as seriously.
Good fortune in making your decision; only you know if the little fella is beyond healing.