You have a pic of the filter?
You could put some sections of plastic grate over it (cut to fit) or place some strips of self-stick velcro across the top......so that the edges stick to the outside of the filter frame/box.

I would try some double sided tape, after touching the sticky tape maybe the little imp will leave it alone. Good Luck, let us know what works for you.
Christine

Thanks, everybody, for the suggestions. It's one of the typical Tetra filters, where you just slide the cartridge in, so it's pretty easy for the cat to pull it out too. Last night, we tried taping a piece of plastic netting over the opening where the filter is. Some of your other suggestions sound better, though. So, if the cat succeeds in getting the netting off, I'll try them.

This cat is a little peculiar as he puts his paws in the water dish and splashes out some of the water when he's drinking from it too. He does have one filmy eye, so it may have something to do with sight problems.

Ive heard they dont see water close up very well - so prefer it if its running or moving. Have you seen those little fountain type water dishes? Was thinking of getting one for my Orange, who lately has been scooping water out of his dish with his paw and drinking from that. Making a big old mess in the process! Not to mention the water dish gets full of crud.

Could you put a piece of tin foil over it, tape it down? Probably could be easily removed, but kitty might be intimidated by it and leave it alone.

I'm sure that fishy water has a lot more flavor than the plain stuff you offer her.

When we had a fish tank, we had a screen top over it so there would be no way for a cat to get into it.