Our cat drinks out of our 2 1/2 gallon betta tank too, doesn't mess with the fish though, just wants the water. I guess he prefers warm water over cold...I don't get it.
iv got an arro n a snake head now n often here my tank lid bang at night but when i was a kid i had goldfish n my cat hooked all 6 out n was just sat watching em die as i walked in