Your clay substrate sounds a lot like Florite.

Reading the blurb, it sounded more like kitty litter to me than Flourite.
The
instructions said to put it under a layer of sand.

I haven't opened the bag yet (in case I decide to return it) but the texture
felt through the bag seems to be exactly like potting soil. My guess is
it's something very similar to potting soil but without the fertilizers and
anything else that could be damaging to fish.

So far I've seen a couple people recommend sand only. Since I have lot
light (15 watts) my plant growth will be limited anyway. I just want a few
plants as a backdrop for the aquarium. I'm very seriously considering the
sand only method, along with some plant tablets to stick in the sand.

Michael