Bring your mag up by no more than 100 points.
If you Ca drops again tomorrow you'll need to "turn up your reactor". I'd try Rich's link for figuring out what you need to increase. I would think you'd need to increase both your bubble count and your effluent a hair, but you don't want your reactor PH to drop below 6.8 if you're using ARM (read the container for specific PH range, I'm going by memory).
If you Ca drops again tomorrow you'll need to "turn up your reactor". I'd try Rich's link for figuring out what you need to increase. I would think you'd need to increase both your bubble count and your effluent a hair, but you don't want your reactor PH to drop below 6.8 if you're using ARM (read the container for specific PH range, I'm going by memory).