What can paralyze fish?

One way to stun or paralyze a fish is to hit it quite hard on the head with a stone or the likes. Another way to "paralyze" the fish is to cut off the main fin [ the propeller fin ] then it cannot swim away or swim at all.

fish usually act "paralyzed" when they are in shock. like most people have said you do need to change the water, again making sure that all of your levels,