A good product to coat it with is sikatop, I myself have used it on a background as have other members here. It's completely safe and won't leach into the water once it's cured (48 hours) it is a polymerized concrete so is applied the same as concrete and tintable. Another thing to keep in mind is the concrete will make your background larger so make sure it's cut smaller than the area you want it by about 1/4-1/2 inch. Also the concrete won't stick to silicone so be sure to wipe excess off. A personal suggestion is to assemble it in chunks, as apposed to one solid piece. This will make the install easier. The little house/caves as one piece the base done as two seperate pieces and so on. This will also allow you to cover up the styro better then silicone the pieces together as you install them.