Where are you? Can you get a product like Boundry or Feliway at a local petstore or mail order? If so you can spray around the base of the tank and it should keep the cats away entirely. You could also tape tinfoil on the top of the tank, cats hate it and don't like to walk on it. You can also keep a spray bottle on hand and give them a spritz when they start to hassle the tank in anyway. There's also a product called a Scat Mat that you can get for your counter tops and couches and things, it should work on your tank. When the cats jump on it it gives them an unpleasant (but not painful) buzz. A lot of cats will learn that that area is a no-no and you only need to do these temporarily or occasionally when they need a reminder.