On a related note:
I personally LOVE my fiance's big headed, brick wall, cat. He's my snuggle buddy, leaves the other beasties (except birds) alone, and endures the torment of having a 3 yr old toddler. He's pretty awesome.
However, I had an issue with him and his brother (who was my cat until he went missing from my dad's) getting into the tanks for the water. Simple solution, lid, duct taped down on the back, which is where they were lifting it. lol
A lot of times people get pets and expect almost human intelligence from something hard wired to act on instinct. Your cat isn't going to think about the repercussions of invading the aquarium, that's your job.
To avoid issues, make sure to provide the proper barriers, or train your cat not to climb on the furniture around the tank. >_>