How do you know that your phosphate levels are low (< 125 ppb)?

If you didn't test with one of these two tests, you probably do NOT know. There are some VERY inaccurate tests being sold. Keep in mind, pool dealers make money by selling you chemicals, NOT by solving your problems. If they DO solve your problems, you won't see them very often. I'm not saying there are no honest dealers, but I'm saying that the ONLY incentive pool dealers have to be (a) honest and (b) accurate is their own moral values. Financially, honesty will hurt them rather badly.

The AquaChek is easier, but only useful below 500 ppb; the Taylor is 'fiddly' to use -- the test cell tends to fall over -- but more functional over a full range.
Hach (AquaChek) 562227 Phosphate Test Kit
Taylor Technologies Inc K-1106 Test Kit Phosphate
Still, getting your chlorine levels up, is the first step.