Most humane way to put him down is to put him in a bowl and put it in the freezer. He will just go to sleep at a certain temperature. Sad to do, but sometimes unavoidable. btw, if you go to your LFS, they'll have a tank with probably 100-200 of those, being sold as feeders, 10 for $1.00 Take him to your LFS if you can't put him down yourself. Adding a 20g or similar for that fish is going to be equally cruel in the long run, because a comet will outgrow most common tanks. Comets can easily get 12" and are best kept in ponds.