you can use fine crushed coral (like fine sand) that will raise your PH quicker and keep it higher than the larger chunky size. i like the bigger chunks because they don't get sucked into the filter so easy and they will raise the PH, but not as quickly and it won't go as high.
i use RO/DI water (6.0 PH out of the filter) in all of my tanks, crushed coral and river rocks in my substrate. and one piece of driftwood in each tank (more for visual appeal than water treatment, but some of them still leach tannins very well). the PH in my tanks runs from about 7.0 - 7.4 at any given time. highest i've ever seen when i test it is 7.6.