IME, Salifert, Red Sea, and ELOS PO4 and NO3 kits are useless. They always read zero whenever I used them on my reef tanks. Regarding the PO4 kits, I used each of them on filthy shark and giant grouper tanks at a public aquarium where I volunteer. The ONLY positive result was Red Sea phosphate kit which showed 1.0 ppm on the grouper tank, which was filtered with decades old debris laden bioballs and rags of blue filter pads.
Regarding the NO3 kits, the zero readings rarely matched the API NO3 kit. The latter kit gets awful reviews, but IME testing my tanks and a wide range of public aquarium tanks, it can and does distinguish between 0, 5, 20, 40, 80, and 160 ppm NO3. IOW, it is a worthwhile and useful test of NO3, and far more accurate than Salifert, Red Sea and Elos kits.
Which leads us to the ultimate match-up -- Hach or Hanna low-range PO4 test kits. For years, I was unable to coax consistent results from Hanna checker. I then tried Hach kit and had no problem distinguishing between 0, .02 and .04 PO4 readings.
But each time, I used Hach kit, I also did Hanna checker test. And after dozens of tries, I feel I have finally mastered the technique of the Hanna kit. Unfortunately, I consistently still get 0 PO4 each time I use it. So on the day after tomorrow (Monday's a work day), I am going to use the Hach 1.0 ppm PO4 solution to test one kit against the other.
May god have mercy on their souls.
Mike
Mike