I'm thinking of making a cement mold out of Styrofoam to get the exact shape I'm looking for. Then filling the mold with cement. Is there a release agent to spray on the mold that would be ok if some lingered on the cement after removal from the mold??
Is the intent to reuse/tweak the mold for future use?
Must admit, I've put a lot of thought into making a foam bg but I am intrigued by the idea of creating a foam mold and then making a cast of it--much less work to install and no worries it'll launch out of the tank like a Polaris missile.
As mentioned something like Pam cooking spray with silicone in it would probably be the best release agent. But I think the bigger issues would be keeping the durability. If you are thinking of making rocks or something like that where the cement will be thicker then that would probably work. But if you're thinking of doing a BG or something where the cement would be thinner than a few inches then cement tends to fall apart in thinner thickness. Even a BG as thick as 1-2" would be hard to hold together on its own out of cement. Usually cement projects need to be pretty thick and even then need some sort of inner support to make it work. Just like with rebar in roads, no inner support and it just crumbles and cracks apart.
...microminiature rebars!...namely: fiberglass mixed into the cement ...it's available in various lengths...I'd say 1/4 or 1/2" fibers would work very well to crosslink an aquarium structure and make it infinitely more durable than without...an acquaintance of mine had to build a cement canoe as a school project!...reports are that it held fine on a lake, but then succumbed to rock impacts on a river cruise...search "chopped strand fiberglass" on e-pay!