Laterite:
Purely a substrate additive. It's a rich red clay. Inorganic, and inert. It's very rich in iron and some other trace elements. Must be mixed with a regular substrate, and buried under a layer of some regular substrate. Otherwise, the iron and nutrients will enter the water column, clouding the tank, and resulting in extremely high iron levels, which will lead to algae problems.
By being inert that would imply that it won't effect your pH or water hardness. I've never used it so I can't say for certain, this is just what I found with a quick google search.
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