I thought one should be fine based on this BRS vid.
That video ended up with two Primes over an LPS 40 gallon tank and three over a SPS one. I have one Prime over a 15 peninsula and it is barely enough. Like others have said, check your calcium alkalinity and magnesium but you will not grow anything more than low-light tolerant LPS with only one Prime.
I doubt your phosphate is really zero and with your water change schedule I think the water is probably not the issue unless you are literally starving your fish.