Also, what is your salinity? The 1/2 cup/gallon mixes up to about 29 or 1.021 SG
(specific gravity). That's pretty low salinity for a reef, I have found that Reef Crystals mixed up at 3 full cups to 5 gallons bring it to right about 35 / 1.026 which is where you want it. That is about 20% more salt mix, so all your readings go up about 20% also (Ca
Just keep that in mind.
How old is your tank? Not much bio-load, so unless you've been feeding lots of flake food, I wonder how your phosphates got that high - seems wrong.
Try testing your tap water and some of your RODI
water for Phosphate. See if your tap water is the source (it probably is, but that's REALLY high phosphate even for tap water)