I've been adding Strontium and Iodine since I've had the tank going on(5mos.)
I have a 55gal tank and never add anything but Kalk
and occasionly some Caclium when the kalk
can't bring it up to where I want it.. I have never and will never add Iodine or Strontium to my tank.. If you are doing bimonthly waterchanges expecially 20% then you shouldn't need anything else as the salt will provide everything that your tank needs.. Also I see nothing in your tank that even should really consume CA
that fast except maybe coraline and snails.. Most of the additives are for people that do monthly waterchanges if they even do water changes at all...