Mine are fascinated by them. They'll sit and watch them all day. One tried to jump on a tank while I was test filling it in the yard and fell in, but other than that, they seem to avoid the top. The fish tanks are just kitty tv here.
Hopefully will have my bedroom tank set up soon.... That might make my cat more interested. My futon is facing towards the tv and away from the tanks. But when my cat lays on the bed she usually faces the blank wall where I plan on putting one of the tanks.