I am buying a used 55 gallon and a filter because I am on a low budget for this tank. I am making the stand and a background like this guys out of styrofoam http://www.aquariacentral.com/forums/showthread.php?t=108995. I am using pool filter sand to cut the cost of gravel. My only problems is rocks/caves. So I have two questions, how I know if the pool filter sand is safe to use and does anyone know where i can get rocks cheap or how i could make artificial caves and rocks?