One other thought. Are you using gfo in your system? If you are, gfo when new will lower the alkalinity in a tank fairly drastically (especially high capacity gfo). Then when it's a few days old, it has little effect on alkalinity. Well that causes a problem when we are automating dosing based on testing. If we adjust the dosing because the gfo has lowered the alkalinity, well then once that effect is gone, we are overdosing the tank. I've seen it and confirmed it several times. It's actually a pretty significant effect.
No, I'm not using GFO... just carbon in a reactor.