The bottom is dirted capped with black blasting sand with a bit of eco mixed in. Top ledges are primarily blasting sand with a bit of eco again. The d. capensis was growing pretty much in plain sand, so I hope it takes to the black blasting sand.
As for the amount of RO I use on this particular tank, it only holds ~27 depending on how high I fill it. I don't use more than 10g of RO water for a major water change (how much depends on if I remineralize or am lazy and just mix in tap). I can do water changes as infrequently as once a month.