My mom had a sign made of limestone in her front yard for years. Her landscaper hit it with his riding lawnmower and broke it up in some nice chunks. On a average, maybe 15 6x6 chunks all about 4 inches thick. I rescued it out of her trash can today. One side of each piece has Georgia red clay on it. The edges that broke are pretty clean, but have some household garbage junk on some of them. The side with the eched sign looks like it's mildewed. How would you clean this and make it safe for an aquarium. I did a rough stacking and it's going to look great not to mention all the ph benefits.