These “rocks” like the above picture shows are held in place by magnets. Their fake rocks made from non toxic and inert ingredients. Each and every piece is different and since they are held in place by magnets you can remove them from yourÂ tank and easily move them around too. They come in a variety of sizes, from large to small, the smaller size is pictured. It can fit on your tank up to 3/8″ thick. Not bad. Retail is rumored in the $35 range.