I wouldn't euthanise.
Then again, that's just me. I take one look into their little eyes, and I just can't do it.
I euthanised a a tetra with severe ammonia burns; it had absolutely no strength left, and could only "vibrate" through the water. However, now I regret it; I put a Festive cichlid into a small tank that had no ammonia, and he recovered.
And in this area lies my greatest weakness; I was about to euthanise a Pangasias with ammonia burns, but I took one look into his beady little eyes, and I lost it. I placed it into another tank, and he recovered. I owe it to that fish to take good care of him now and get him a good home. And I will.
-Lynden