My thoughts on fish euthansia is wait until there seems to be absolutely no hope. As in no swimming or eating for a prolonged period. A general wasting away is a good indicator to me that quality of life is pretty much gone. And then I use water with lots of ice in it if it comes to the pt of euthanizing. It works pretty fast. I really hope you don't have to do that! Hoping the best for you.
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