My levels are perfect.. why?

[Spyd]

Since you have primarily softies and LPS, you don't need much in terms of Cal / ALK / Mg. Yes, the clam will absorb some as will the LPS, however, every time you are doing a water change you are adding all 3 from the salt mix. I highly suggest keeping up your regular maintenance of water changes twice a week as you don't want your tank to get too dirty and it is adding the big 3 additives every time which will help keep your levels steady.