To be honest, I appreciate the way the natural world works, as there is balance; however, the way a lioness hunts and slowly suffocates a baby gazelle it catches via piercing and crushing it's breathing pipe is immoral in terms of humanity. But then again, humanity, to me, is another way that people desperately try make themselves better than any other animal that we co-habitat with on this planet.
Compassion for life though, is a trait that many animals have, however many animals also don't have that compassion. Animals, in general, don't kill another unless it's for territory or to eat them. People however, kill for every reason imaginable, whether it be for bragging rights, sport, food, to eradicate a 'pest', etc.
I'm glad to see that people here are trying to be compassionate about the possible pain that a fish may or may not experience while being euthanized. I believe that if an animal can feel stress, it can too feel pain. I do not have any evidence to back this belief up, however, the way I see it, until it's proven that fish cannot feel pain, it would be more moral to assume that they do feel pain, as we do, since we do not know any better nor can we recognize the ways in which fish may or may not express pain.
But that's me just going off on a tangent.
Back to the original post...
If I have a small fish, that say is deformed in a way that will make it much harder to compete for food in what ever miniature ecosystem I have provided for them, which would therefore cause them to slowly waste away and starve to death (not to mention if those deformed fish also do happen to be lucky enough to survive and breed and create offspring that are also deformed in a manner that does not help them survive), I would simply feed them to one of my predatory fish. Nothing is wasted, the deformed fish served it's purpose as food, as it would in the wild.
If I have a larger fish that has gotten sick, and is clearly showing signs of it not being able to compete for food, nor is it trying to eat or survive, at that point, I would simply cover the fishes head, and decapitate it with a sharp knife. The rest of the fish does not go to waste, it gets filleted, and fed to my turtles. The death is simple and quick, although a bit gruesome for the more faint hearted, but at least that fish that was sick now can continue to serve a purpose, and will contribute to the hopefully long and healthy life of another animal.
This is the way I think of it. I do consider my fish as pets, but at the end of the day, the method used to euthanize a fish is irrelevant as long as the death is quick, and the remains of the fish are not wasted.