The litter I used, as specified, is called Special Kitty litter. It is plain clay, no additives. I have heard though, that west of the mississippi there is a different clay mine, so you should test it for pH/kH changes before using it in your tank. Just put a bit in an empty bottle, add water, test it in the morning. Also, I don't suggest using gravel, as there is never a compact seal over the clay. If you really want gravel as opposed to sand as a top layer, put it over the sand layer, not in lieu of it.