I just purchased a big bag of scoria, based upon what I've read here, primarily from you, SouthPark. I'm intrigued by it, and I've got a few Catts that need repotting, and I'm going to try them in scoria. I ordered the medium scoria, because I use medium bark for my Catts. Does that sound right, or do I need a different size to account for the different properties of the medium?