Listening in curiously in since I am also in same boat. Times two.
I just bought a Hanna Phosphate kit yesterday, since I couldn't get a reading on my Salifert kit. Figured I would get a reading on the Hanna. Nope. Hanna also read 0.00 ppm
Read in Hanna manual that accuracy is +- 0.04ppm +-4% (So if I'm between 0 and 0.03, I could potentially get a 0.00 reading). Did I understand that correct?
Was considering the Hanna Phosphorus tester instead, which reads in ppb, but was advised not to go that way.
Also got a Salifert Nitrate Kit, since wanted to also check this.
[TANK#1]
65Gal (SPS only) Frag Loaded 9mnth old tank, which has No algae problems.
SPSs are doing fine. Very nice polyping after I got all parameter stable just recently. 3 fish.
Phosphate 0.00
Nitrate 0.0
[TANK #2]
90 Gal (Mixed Soft/LPS) 15 yr old tank, which will be rebuilt (running Fallow for a few weeks due to all fish dying).
Tank is Wrapped DARK to kill off all (Cyano, + Green Algae problems) .
All Corals out and in a holding tank. Rocks are sparkling clean/algae free after 2 weeks in darkness. Some Coraline loss.
Phosphate also 0.00
Nitrate 5.0.
Reason for interest in both Phosphate & Nitrate is when I start-up Tank #2, add corals back, and start with fish again.
Hoping my past Algae / Cyano problems will be gone (with many adjustments I'm making; Improved Circulation, added GFO reactor, Rocks scrubbed, cleaned, and starved from light).