If you have algae, it is feeding off of nitrates and phosphates. That is why you have a reading of 0. You can't pull it out because it is strong from the food source in the water column.
Are you using ro
water? Are you feeding flake/pellets? How often are water changes and how large?
Your calcium is low most likely because of your alkalinity being high, mag being out of whack, or that you aren't using a reliable test kit. For these tests, Red Sea, Salferit, or Hanna checkers are the only reliable enough. That said, why are you testing for calcium when it is a soft coral dominated tank?