My phosphates also build up over time through fill water, about 300 ppm right out of the tap. When I can't keep FC in the pool more than a few hours I have it checked. The last time it was 3000+ ppm, about 3 years worth of build up.

I have found that ALUM (purchased from Leslies) is better and cheaper than No-Phos at that level, as it lowers it almost overnight. The drawback is that it's a mess to deal with. It totally clouds your pool then settles on the bottom overnight. Then you vacuum to waste, and again a day later after more settles...