Just want to make a quick guide on how to cull a fish. Culling a fish can be quite devastating for most people - especially if you're not used to animals dying. But, if you're fish is really sick an about to pass, culling can be the best option; it puts the fish out of suffering. I was forced to cull 3 fish tonight (Mountain Minnows) and it is definitely not fun.
Freezing is a commonly used method for euthanizing warm water fish. To freeze your fish, freeze water in a small bag until the ice is slushy. Then place your fish in the water and continue to freeze it.
The fish will probably feel something as you slice the knife in; even it is momentary pain. At least this method is quick (as long as you do not hesitate once you start the task).
Carbon dioxide can be used to euthanize fish. Alka Seltzer is a popular source for carbon dioxide to be released in water. This method may seem innocuous enough but most people who have used this method say that their fish thrashed around a little bit just as if the fish were out of water. You may need to decapitate or freeze the fish afterwards to ensure death.