Ah. Well, getting colder and going to sleep, essentially, versus the burning of the gills from the tap water and the waste that they wind up in. Acid to the gills, essentially.

The most painless and swiftest would be to get them into a baggie and slam their heads against the counter...but I personally can't do that. Yuck.

As for it being "a fish", I'm thinking you know they've got more personality than "just" being a fish. It's just an opinion, but I think they deserve as gentle a death as possible if we have anything to do with it. :)