I used to walk into the living room only to find my cat perched on top of the aquarium with her paw all the way in the water slowly swishing around trying to catch a fish.

Cute but annoying.

I used a squirt bottle with water to disuade her, it trained her very quickly. She was too fast to be caught and/or spanked... but the squirt bottle could get her every time.

I recommend the squirt bottle, but you have to see them in the act for that to work.