Ph fluctuations tend to rise first then drop. Has to do with nitrites and nitrates, the breakdowns of organic matter. Hard water with high amounts of calcium and/or lime tend to rise faster but I dont know why. I am a smoker and a grower, not a chemist. Weekly waterchanges and large resevoirs is what I mostly use to keep ph levels in check, since I pretty much do the same routine with everygrow I rarely check since I rarely need to adjust.
Hope you like the product, it is pricy but I seem to get very good results. Suppose to be the best enzyme product on the market.