How are you testing? With the Aqua-Chek, or the Taylor? ( I scanned back very quickly, but didn't see)
The reason I'm asking is that I found the Aqua Check and the Taylor matched fairly well below 1 ppm, but not above. At 1 ppm or higher, the AquaCheck made no distinctions -- with samples with 1 ppm and 3 ppm PO4 appearing identical to me.
If you're using the Aqua Check, you need to dilute your test samples to results of 0.5 PO4 or less, and then multiple back. (Mix 1/4 cup fill water with 3/4 cup distilled water, test, then multiple x4).
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