I don't your think your tank will drop down to 3 dkh in what I assume to be less than a week. It's certainly not a linear relationship and your corals don't just suck a certain amount of calcium and alkalinity not matter what is available. Your levels will be low when you return but not deadly as you assume. Your pH controller should also prevent your other scenario, even if I opened my needle valve all the way the pH in the reactor would still not drop that low before shutting off the CO2 off and then it would simply stay off or longer, how you can dump 20lbs into it is beyond me.
Anyways just something to consider for next time, I've had a fully stocked SPS tank before that ran on a CA reactor that required more attention than I was comfortable giving a tank sitter. I would shut the CO2 off before I left and adjust levels when I returned. Even after two weeks nothing was critical.
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