I went to The Home Depot and looked at rocks, slate in particular.. I wanted to add another level to the rock structure in my tank. for $10, I got about 25 pounds (7 slate rocks). It was a great deal! I did windex them all, scrape them with a stainless steel brush, boil them for 1 hour, windex and scrape them again, and boil them for a second time.. No residue! No micro organisms! No cloudy water! Tell me what you think...