introduce new friend, Pete the piranha
i had a fish like that once , it was a goldfish and went really fat like it was going to explode , i just left it and a week later i found it floating in the pond
Kill a fish instantly by pushing their eye in.
*edit*
I'm being totally serious.
Ignore all of above.
I have a fish keeping book which says, ice water is best way to euthanase a fish. Lots of Ice cubes and water, fish will just shut down, and will not feel anything?