In our house it's the dog. He just loves drinking the water during water change the tanks on the floor at the LFS. (Luckily they are just feeder goldfish and the staff do not mind if he does this). The cat will just ignore the fish though. Some really great ideas here. It sounds like your problem cat is a kitten, so I would use the dunking method to teach it a life long lesson.
This little mofo is still at it. He bats at the output of my HOB and then puts his little wet feet all over the front of my tank making beautiful streaks.
The foil worked for about a month until he figured out it was harmles, would knock it off the top of the tank and go about his normal routine.
I've put more thought into dunking him and seeing if that will make him lose interest.
I say dunk the little guy! I can'ti magine that NOT doing the trick.
I have a 5G divided female betta tank sitting on my kitchen pass through window shelf. It has no lid, just frog bit. Well both my cats have decided this is the best source of water in the whole house. I have to top it off with about 2 pitchers of water A DAY. They don't seem to notice the fish though so I figure hey, I'm happy their drinking (one has urinary tract issues so this is GOOD).
drawback? One of the females has learned that anything at the surface of the tank means food is coming. she swims right up to the tongues!! I'm scared that one of these days one of them is going to get a tasty snack, by pure accident.
but really, I think the best option is to get a secure, locking lid, put the chair back, and let the cat enjoy the tank like you do. As evil and insidious as cats are, they will find a way to get at the tank. So if you can't beat em, accept defeat.
I'm a dog person, my dog will sit in a chair and watch my 20g. He's enthralled by my Opaline gourami. When I lived with a cat, the cat would do the same.
There is something called Sticky Paws (http://www.petsmart.com/product/index.jsp?productId=2755192) that is basically double sided tape. Wide double-sided tape from an office supply or hardware store or hemming tape from a fabric store would probably work just as well and it's cheaper. I would make sure the hood is secured to the tank so the cat doesn't take the hood with it when it tries to get away. After my cat knocked over a tank and killed a betta (my first and only fish at the time ) a few years ago, I covered one side of a large piece of cardboard with it then put my 2 1/2 gallon tank on it. She figured out pretty quick to stay away from the tank. She gets into trouble everywhere else in the house but never bothers any of my tanks, even the ones with flashy fish in constant motion. Also, most cats don't like citrus smells so try putting something citrus scented on or near the top of the tank that won't harm the fish.
petsmart also carries another wonderful deterrent item called "ssscat", which is a canister of compressed air with a sensor attached to it. anything moves in front of the sensor and changes how much light it detects, it depresses the valve on the can of air, releasing a loud stream of air in a sound that cats HATE. works like a friggin' charm. ^_^
(this thread also makes me very grateful that my cats have no interest in my fish or my turtle. my husband wouldn't let me get fish for years because he thought they'd just be expensive kitty treats, but in three, four months, all they've gone after is the fish flakes and turtle pellets, cuz they smell like food >.>)
This would work but a boom box? I say get an iPod and a good Dock System with a remote when he get ontop just push play for the dog bark or any other loud scarey sound.
I would dunk the whole cats face in there that would be my entertainment just make sure you got full control of its rear so it dont twist around and shred your arms up. Use to have to hold the cats down for my sister for there bath. They eventually stopped fighting when I was around guess they knew there was no escape from me.