Hello. Our LFS has a nice selection of slate and lava rock and a couple other types of rock. In my sculpture class, I have to make a sculpture using non-traditional materials and I was considering constructing something for my 29 gallon tank. However, I'm not sure how rocks like slate, granite, lava, etc. affect water parameters like ph or gh. Could anyone help me with this? I'd like to compile some sort of a useful list that myself and others could use for reference. Thanks!