Most of the stuff from my sandbed that vacuuming removes is super-fine white/grey dust/silt. It is almost entirely soluble with HCl and doing a Ca test on the dissolved stuff points to it being around the expected Ca % for CaCO3.
Why the dissolution of aragonite sand under mid-7's pH conditions in my sandbed leads to the endpoint of fine CaCO3 dust I dunno, but probably too far an aside for this thread :)
I've never used a Ca Reactor, but my guess is that with the pH in the reactor going much lower than in my sandbed, the mush left behind is likely acid-insoluble - and thus maybe not like most of the the dust/silt that accumulates in my sandbed.
