My water is really hard and I have a ph of 8 out of the tap. I have dirt capped with black diamond and 2 driftwood stumps in my tank and it hasn't changed a thing. Ph is still high and hardness through the roof. Best way I have researched to lower ph is by cutting your tap water with R/O or distilled water.
MGOCPS, will help. but is is only a short term fix. it is loaded with peat moss and wood chips. So if you do not sift it and use it as is, it will help buffer PH down. But only until all the stuff has run its course. Depending on how much you use could be a month could be years.