you have corals?

Yup, probably 60+ frags and 8 colonies of SPS and 5 colonies of LPS in the tank. Plus a bunch of oddball corals.

Are you getting any precipitation of any kind on your pumps? I had a problem on my last tank that involved me constantly testing really low levels of calcium, which in turn led me to up the rate I was dosing. THe problem was actually that I was dosing in the wrong ratios, putting in too much alk. It was driving a precipitation reaction. I only figured out what was going on when my pumps all failed in the same week.

Can't see any kind of precipitation but ill pull some pumps out and double check.

I would say (with a fair bit of confidence) corals are not able to pull Ca down to 240ppm (please correct me if you think otherwise) ... I let mine slide down to 300 a while back and was losing frags by the handful but I may have had additional stresses involved. Also, I don't think Ca is going to ppt out at levels anywhere near that low, unless perhaps there is something else going on (like large pH swings from the Kalk).

I am going to throw an idea out for comment to the group: what about an electrical ground? I don't remember my chemistry well enough (repressed memory I suspect) to say if a charge on your tank would promote Ca precipitation. A cheap (maybe $20) grounding probe should be available from your LFS and might be a good idea regardless. For the amount of Ca your tank appears to be consuming you should see it on everything! You can also check your pH upon addition of your Kalk (or anything else for that matter) to see if the swings are substantial (maybe you manually replicate a typical dose if you don't want to wait around for the ATO).

Dan