Don't bleach, think about drowing in bleach and lighting yourself on fire, bleach burns fish's scales.
I have read up on a bunch of articles on here on stuff like this and while it sounds gross blunt force trama is quick and effective.
But the easiest way I found and my prefered method is freezing. But there is a right way to do it. Puting the fish in water in the freezer is painful to the fish and inhumane this is probably the one you read about. As the water freezes the ice forms crystals and is very painful to the fish.
The humane freezing method is take a glass or container and fill it with water, float a lot of ice cubes in it for a while or put it in the freezer. Pretty much get it as cold as possible even to the point of the surface freezing over. Break the frost off the top to expose the freezing water. Putting the fish in the water you are going from 70 degree water temps to 30's in a split second. It puts the fish in immediate shock, think jumping in cold water times 50. The fish doesn't suffer at all and has no pain. It's literally over in less then a second. Let the fish sit in the water for a while to be sure the fish's heart has completely stopped and dispose of him.
Just remember slow cold is what is painful, immediate cold leads to shock and immediate death.