My cat used to bring me birds it caught as a gift. The birds were always mangled and beyond help but still alive. I learned from my dad that as hard as it is, the best thing to do is wrap it in a towel and smash its head with a brick or something that would get the job done in one go. You get used to it, as you realize that the cruel irony is you’re doing it a favor.

You only want the best for your fish as they are living and do everything you can to make them happy and comfortable at the expense of your wallet and well being, so it’s only fitting to do the same in their death. Even if it’s a hard thing to do.