I'm not sure about whether the additives in grout are toxic to fish? I've heard they are, but don't quote me on it..
A lot of people use cement and sand to coat foam for underwater backgrounds. You just need to cure it for up to 28 days, then soak it for a few weeks to leach out the alkaline salts.
I've made fake rocks out of just foam coated with epoxy, sand and powdered soil. They look pretty natural IMO but they are super lightweight so you'd need to silicon them down very well..