I very much appreciated post #25 and as a former chemist, appreciated your explanation. I should point out however that you leave some things out. As you know, there are mutiple reactions going on here (CO2/Argonite, Coral/Water, Percipitation on surfaces, etc.) and the system is overall, quite complex. CO2 is required to "create" calcium and alkalinity. If CO2 is the limiting factor in this system, then every time your corals take up a bit more, you need to adjust. (Water that "needs" cal/alk goes into the reactor, but the pH in the reactor has gone up because the CO2/Argonite reaction has exhausted it, so you add CO2). If there is excess available acid, then the system has some stability built in. Because CO2 only partially dissociates in water, there is, in effect, an excess of acid present. The CO2 buffers the system against changes. That all said, God help us all if we're going to rely on chemistry I haven't looked at in 15 years, so forgive the errors. Regardless, after reading the thread, I felt like I hadn't read a clear, English answer to a very interesting question.