Actually I have the h 713 phosphate low range tester. I find its sort of hard to use. You have a very limited time to get a very small amount of reagent into a small perfectly clean glass tube and into the checker before it times out n shuts off or takes the reading and then self shuts off. its one of those things you can't do when your distracted and you have to have it all set up and ready to go before you start the test procedure.

I find it almost always says 0 on all the water tests, occasionally .02 i believe i have seen, on the test which makes me question the meter sometimes. But I do not have phosphate related issues in any tanks that i know of.. more likely lack of actually. I don't completely trust the unit i don't think. maybe some day i will send my water out to be tested at a test facility when i really think i have hit that sweet spot. that way i know what such a spot should be. I'm definitely finding the way the corals look Vs the way there supposed to look is a skill that takes time but speaks volumes about what is going on in there. Having all the test kits and doing them regularly helped me link what i see to what the issue is much faster over the past few years.

So far i always read zero's on the nitrate and phosphate tests which has lead me to this topic. Ultimately i may have to dose a small amount of p04 as well but i believe the ultra low amount would be much harder to manage effectively VS the nitrate at 100X as much being tolerable Vs phosphate levels. most of this should be coming in with my food and even some in my water change salt. Which leads me to a brief point of what your salt mix has in it.. in my case i would likely benefit from a salt mix that was actually real sea water and contained 1 ppm of nitrate and .02 ppm of phosphate in it Vs what is sold as salt mix with no phosphate or nitrate in it.
I suspect thats why at the reef shows they use the jugs of sea water. As well as the convenience of it. but its actually as close as you can get to correct sea water and only small adjustments for regional salinity for certain coral regions would be needed if anything at all.

Over all I'm already nearly sure dosing nitrate is helpful for my system. Its pretty obvious already something good has changed in multi tanks. Though only one of them i am dosing consistently "the sps system". the other one i dump in a bit every once and awhile. I don't have a second doser system dedicated for the lps tank yet.

I actually dumped in a chunk into the big 300 to start getting the nitrate up a bit out there as well. I'm sure its all 0's as well and right now a small test bird nest coral branch peels off to white in about a week or two. I don't think its light, i think its either a pollutant in the KALK mix I'm am using, which i got from a break down tank system and it smelled bit funny to me "like your grandmothers laundry soap sort of smell" or to low nutrients. They don't ever brown at all they just peel off n die. more testing and some more time will help me dial in the cause out there. i ran a bunch of carbon on it so i think its more likely low nutrients too new "Clean" water.