I'm in agreement with your tank being pretty lightly stocked in corals to be seeing such drastic swings. I would say your alk
is low for the same reason mine is...if you figure it out let me know! Lol...
What I would do, test your alk
and calcium right after you mix up your water. That will give you your baseline. It is just hard to believe all your stuff is being absorbed when there isn't that much in there to consume it. My calcium stays steady at 595 and my kh
is always at 80 (breaks down to about 5 or 6 I think if the math is right), but this is with Kent salt mix. My point is that if it is stable and you have corals growing, then just let it go. Our fiddling with it can cause an issue, when there might not have been one to begin with. My test results are from Hanna checkers.