Do reef/ fish tanks and cats mix?

[DanG]

My GF has 2 cats, one is too fat to get up onto my tank, but the older one I will catch on top of the canopy from time to time. He gets 8 shots from my nerf gun as he is running away. He knows he's not supposed to be up there, but that doesn't stop him.
I Keep the nerf gun on the bookshelf next to the tank in hopes that it keeps him away, and it works for the most part, but still far from 100%.