I've used clove oil, put a few drops in and they'll get sleepy and eventually fade away quite peacefully. It's a bit like giving them a sedative and they just don't wake up. Only use a few drops. If after say an our they're still breathing then add a few more drops. Oh put them each in a separate small container, and maybe cover the containers up so they're in the dark... Help them to relax.
I only did this when I had no other choice, with a betta fish that had a huge cancerous lump.