Mike Wise writes: << Fluorite is considered a Halide mineral, like salt. Unlike salt it > isn't very soluble in water. If it was, we would have places dissolving in > hydrofluoric acid (used to etch glass!). Pure fluorite shouldn't alter the water > chemistry much if at all. >> True, Mike, but as this is a heavily planted tank, I think she is talking about the substrate additive from Seachem called "Flourite", in which case it is a laterite clay. Very poorly named, as geologists and rockhounds on the Aquarium Plants Digest mailing list keep making the same mistake. Bob Dixon