Maybe it is just me, but I don't think it is a good idea for the cats or the fish to let them drink the water from the fish tank. If I am all alone here, please ignore me.
The Petmate fountain is a silent fountain, it doesn't make any noise unless it needs water, or a new filter. Some people have found this works better for skittish cats. And it is clean. They aren't that badly priced.
By letting them drink the "fish water" isn't that encouraging them to drink directly from the tanks? I mean doesn't this put your fish in danger, and for some reason the idea of drinking the water from the tank bothers me for the clean factor, unless you clean your tanks quite frequently there is bacteria and general ickiness going on. I don't know. Not to mention that is technically the fish bathroom they are drinking from.
I don't know. I think I would try to exhaust all the other possibilities before I resorted to the tank water.
Again, just what is going on in my head when I read this. Please feel free to ignore it