That effin rawks vicious_fish, I've used the same method but covered it with shredded coconut fiber mixed with acyrlic concrete hardener. You can do one for use with fish by coating the foam with concrete and acyrlic hardener and some people even take the time to paint them and recoat with hardener again. Google it, it's the shizz. Theres even a recipe for making fake live rock out there somewhere, I've done that as well and used it in my reef tank when I had it.
When you use concrete in fish aquariums really take the time to follow all the suggestions given for rinsing it. Otherwise you will have a major spike in alakalinity.
Google stuff like hypertufa too. If you read the info on making fake live rock and the info on making hypertufa it will definitely give you some awesome ideas for making realistic fake rock caves etc.
You can sprinkle sand, gravel, shells on the still wet product to add texture and interest, you can mix it right in your concrete mixture too. I have stuck pieces of slate, branch's, shells, etc in the foam or concrete while wet to give a more natural look.
Also have taken plastic planters zipstripped them to a white plastic eggcrate with an eggcrate base...sort of a frame...then sprayed great stuff on it till it vaugely resembled a tree stump with the planters hidden inside then coated it with coco fiber. Think I stil have one of those...if you want I can post a pic.
The only limits are your imagination.