To Test Your Sand...
Get some ph minus Hagen (Sulphuric acid) or muriatic (use to clean bricks and patio stone) dry some of your sand drop some acid on it, if you see a reaction you get some kind of buffer that dissolve slowly in youir water.
—> if yes change your substrate
—> question — RO water is not supposed to be lower than that like 6.5, 6.8?
—> Did you test your KH, higher your KH, Harder it will be to get the PH to drop.