After reading all this I am ridiculously happy I have not had to dispatch any of my fish as of yet. Knowing how long lived some speices may be and my minimalist approach to stocking, I hope to now have to do so for quite a long time. It is nice to know however that there are options as opposed to just letting this fish die on its own in the tank.
My question would be, how do you know when? My clown layed on the bottom for two days, barely moving, labored breathing. I had done all the treatment I could at that point and I refused to give up on him. My coral beauty stayed with him the whole time. Swimming up to him and gently nudging him to swim. It was really amazing to watch. (I say 'him' but I realize as the only clown, nemo is a girl at this point LOL) He recovered fully and is doing great now. Makes me have many mixed feelings about a flame angel I lost before that incident. Could she have made it? Did I do all I could?
Lots of questions in this hobby with just as many answers. Euthenatia is hot topic for all things in genral I believe anyway.
On a personal note, I cried when I lost the flame angel. It still makes me incredibly sad to think about.