Have you ever wondered why proponents of ULNS systems like Zeovit and others bring their nutrients to or near u detectable then add supplements to increase coloration & growth? The idea is to create a working standard baseline then dose nutrients to keep the coral healthy.
I'm not talking about huge levels, but .03 ppm
& .25 - 1 ppm
is ideal. Any more corals will start to brown (reproduction of symbiotic algae), much more and it inhibits growth. But if you go much lower the symbiotic algae will begin to die off causing bleaching events. Most any algae, like plants, requires both nutrients AND light to survive.
What you might read as 0.00/0.00 on your test kits is only a measure of what's in the water column at that very moment. That doesn't mean you don't have NO3
in your system feeding your corals' symbiotic algae. You are lucky to have export & coral uptake very efficiently nearly as efficiently as its added.
Your tests are also location dependent. Most tested water is at the surface in an area of high agitation. If you tested at the sandbed or from a cave on your LR
you might detect slightly higher levels which become dilute or up taken by the time you take your test sample.