Cats, fish, parasites

[Corty]

My cats have a tendency to get into the Goldfish tank and drink the water. Are there any sort of parasites that might cause problems for them? I'm also concerned about the possibility of problems occuring in the event of a cat eating one of the fish.

Am I just being paranoid here? What are some possible risks, and symptoms I should look out for?

 

[Menagerie]

I'm also concerned about the possibility of problems occuring in the event of a cat eating one of the fish.

The fish will die. If the cat decides to eat one of the fish, the fish will be swatted from the tank, so I wouldn't worry about the possibility of a decomposing fish in the tank.
Other than a dead fish, there's not much to worry about. Parasites are host specific (mammals vs fish).