I dose NPK once a week on water change day but, dose CSM+B and DTPA (4:1) on a daily basis just to make sure Fe is always available. My water has DKH of 6 and a total hardness of about 130 ppm. This has been working fine for me and since I dry dose, it's not to complicated for me.
Well, the "total" iron level in the water column definitely persists. I can also buy reagents to specifically test for Ferric ^3+ or Ferrous ^2+ iron, if that makes any difference. Is one better then the other? Or is one not bio-available? (ferric / ferrous?)
Worth at least mentioning in this thread that I have tested & observed that UV exposure will remove the iron residual from the water column.