I have 2 cats and 3 fish tanks. They leave the 10g. alone (most of the time, there is the odd time when one sits on the top, dips her paws into the water, then pulls it out and licks the water off...thirsty i guess??) so that one isn't a major problem. The 20g. however sits right beside the old piano because I had to do some moving around to make room for the 50g and it sits right level with the piano. So, if you put 2 and 2 together, fish tank beside piano= easy oppurtunity to get on top of the fish tank...i can see it all right now: cat walks off of piano onto tank, gets freaked out by...whatever happens to freak her out, leaps off of the tank with terrific force, tank teeters only to eventually fall over, 20g. of water spills all over CARPET and fish are gasping for air. Oh, this all happens while i'm at work for 8 hours and then i come home to a indoor swimming pool! So, to fix this i put a piece of packing tape across the lid and sat and waited for one of them to jump on there. Sure enough, the one jumps up there and i know you're probably thinking cats + tape = disaster? Wrong, her paws touched the tape and became stuck, she freaked out and jumped down (this is why i sat and watched, to make sure the tank remained standing after she leaped off), ran around the house with the tape still stuck to her paws until i caught her and took it off. I also do the same with the 50g. and i've never had a problem with them jumping on it, and even if they did, it weighs considerably more then 14 pounds so its pretty sturdy. Too mak a long story short, packing tape across the hood works wonders! They do it once and when they see it there, they don't do it again. Also, its not like you can really see it or it looks bad on the tank like aluminum foil might. Its clear so its not gonna make the rest of the tank look bad!