Black basalt sand is cheap in Hawaii but in a jar of 50/50 sand to RO water (7.0 initial pH) it raises the ph to 8.5. I am doing a high-tech tank with mineralized soil (my initial soil pH is 7.5). I would like to have pH in the range of 6.5-7.5 and am concerned about the sand. Thoughts? Not sure why it raises the ph so much but was thinking that perhaps included minerals might be affecting it. How about an acid wash with lots of rinsing to try to react those minerals? Or underlying a peat layer?