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.
Video update on the paludarium from September 2014. I replaced the d. capensis with UG. I might try some drosera in a few different spots up a bit higher on the ledge. The c. spiralis is also trimmed way back in this video:
Yes, it is a flowering crypt. pontederiifolia. That is probably the 5th spathe that pont. has put out since spring. A crypt. wendtii bronze is also blooming on the far left, but the spathe is rather bland and blends in. I am working a few new crypts into the ledge I would like to try to get to bloom as well: ciliata and cordata.