Turface (baseball infield conditioner) is a fired clay product, like fluorite, but cheaper (something like $25 or $30 for 50 lbs, if I'm remember correctly). I used their "champion brown" color (which is a medium brown in an aquarium - not dark) for the first few years I had my planted tank up. I used it to cap a dirt layer, but you can use it on its own, too. It's distributed locally at Ewing Irrigation, which has locations around LA, though call your local one before you go to make sure they actually carry it:
https://www.ewingirrigation.com/