Being an aquarium enthusiast for many years I have killed lots of fish. The most humane way is to put them in a bowl or bucket of water and put them in the freezer. Their metabolism slows down until they die. Either put them back where you got them or freeze them.
It is very irresponsible to release these unwanted fish into a local creek or river if you don't know they already exist there.
Fish don't have feelings! Eat, poop and make fry...That's about it.
