I'm sure I may get some flak for this. My current cat of 10 years was caught by me several times trying to snag a fish with its paw on top of a 30 gallon I used to have about 9 years ago. Yelling, squirt bottles etc worked for about 20 minutes until my back was turned again. I finally lost my temper, grabbed the cat by the scruff of the neck, removed the cover with my free hand and stuck the top of her body right up to the shoulders under the water for an instant. She hasn't even looked at any of my tanks since.