My fish has been fighting a tumor that is affecting his gills and making it hard to breathe. I noticed that he seemed to be in discomfort a month ago, and he's worsened to the point that the only time he ever moves is to flinch when I turn on his light to check on him. I've been expecting him to die at any moment for a week now, and I feel like it's time to put him out of his misery.
I know there is controversy over this, but what is the most painless way to euthanize a fish? I was going to drug him, but seeing as he's already struggling to breath, would it be better to chill his water?