Back in my freshwater days I had a 20 gallon tall aquarium with a lid. My cat decided that the warm fluorescent tube made that lid a good place to sleep. One day he jumped up there and the lid gave way. The result: One wet, unhappy cat and some freaked-out fish. I was afraid the light would zap the tank and the cat, but it didn't. I did pull the plug as soon as I saw the lid collapse, though. That cat has never messed with fish tanks again, covered or not.