I agree with CMG that lower animals do not process pain the same way mammals do (that means wolves). Sure they have a nociceptive response to painful stimuli, (ie being poked, burned, dosed with a chemical etc). It's like when you burn your finger with a match, your response is to pull away. Fish and other lower animals do the same thing to a painful stimulus, they get away from it. Lower animals don't have the capacity to reason about lingering pain like humans and to a lesser extent those mammals with a smaller frontal cortex. In other words they don't obsess or dramatize pain, They go on surviving. People that say they have a high tolerance to pain have a different perception of pain than one who has a low tolerence of pain. This is because of the human brain's highly developed frontal lobe. Fish do not have a frontal lobe in their brain.
For me, euthanizing a fish is not about pain but more about taking the sick animals life in a dignified, compassionate and human manner. Some of you may clobber to death your suffering grandfather in the back yard, if it was legal, but it wouldn not be dignified or compassionate. It's easy to justify barbaric practices hiding under the umbrella of "put it out of its misery" and "Whacking" or "The old fashioned way" but there are more humane ways of euthanizing a sick fish.
I find it interesting how much emphasis we put on keeping our fish alive and how little of taking their lives!