I would like to try to make a 3d background using plaster of paris. (I have a 20 gallon freshwater community tank) I know that plaster can affect water parameters so it is not adviseable to put plaster in a freshwater tank. What I would like to know is if mounting the plaster on a thin piece of styrofoam then coating the whole thing with some sort of transparent acrylic sealant would make it "safe" (meaning not affect either water parameters or the health of the fish). Anyone know the answer to this or better yet, know of a specific product which can be used? Thanks.