If it is truely in the water supply you will have to run a RO/DI unit for your water, which you should really be doing with a reef tank. You can pick them up off eBay for under a hundred and they pay for themselfs real quick.
Another thing to do is run a Phos reactor, but that is costly and I would get a RO/DI unit before a reactor.
What is the PO4 reading from your water? also what is the PO4 reading in the tank itself?