What kind of numbers are you running for PH, Ca, Alk, and Mag?
I just spent a couple of days cramming buffer and acid, Ca, Alk, and Mag equations and ratios into my brain. (Thanks Jon!

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If you are having difficulty maintaining levels, you may have Organic Acid buildup, too high of a Carbonate to BiCarb ratio, or just a plain old imbalance of some kind.