You are right, I have not been dosing anything - 2-part or Kalk - nothing.

I really think it is the salt I am using. My other parameters are not crazy so I don't think that is driving it. I understand that if pH is off then you can get some weird calcium readings but that does not appear to be the case.

Check out the poll I just put up - totally forgot to include the Fluval salt (brain-fart) even though you inspired the post!