I use the Hanna phosphate checker and found it to be very accurate and consistent. The trick is to be sure your vial is clean (no fingerprints or smudges) and mix the colour solution for close to the max time (3 mins). Also be sure to press and hold the Add C2 or you won't get the full 3 min countdown. If you just press and release, you will get a reading but it may not be accurate.
Like you, I found the other chemical tests reading zero while the Hanna read higher. But as my phosphates dropped further, the Hanna also consistently read zero. So I believe the Hanna.
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