I am honestly not sure that for anything "low range" the Salifert and Hach tests are much better than the Hannah egg... in my opinion color tests are good when the indicators go from one color to another (eg titration), not so much when it comes to shades.
Other than that, it looks like there was a known issue. Always completely test the first batch of water you make from a new box of salt, no matter what the brand!
Today I test PO4 all day long, I use Hanna HI 713 (Egg)
Tank (220 Gal. + 90 Gal. sump)
Tank PO4 0.00
RO/DI PO4 0.00 ( 9 stages, one is fill / Rowa + 2 Membrane total 11 Stages)
Make up water on Brute container store for 24H already, PO4 0.00
Mix Sodium Chloride at 77F temp. PO4 0.00 I use Korallia MAG 7
Mix .Magnesium Sulfate test 3 times Po4 0.04,0.02,0.02
Mix. Component A PO4 0.05
Mix. Component B PO4 0.11 test after 25 min. PO4 go 0.12
prattreef said:Although we take steps to lower phosphate levels of our liquids, we get a very small contribution from the USP Magnesium Sulfate we supply. (...) Unfortunately earlier in the year (before May) we may have had a batch or two slip through our quality control and fall within the 0.10 to 0.15 ppm range. The source has been identified and corrected.(...) I'd also like to know the batch number of the Component B liquid.