Hi, I have similar "problem".
I use reef salt and ProbioS, ProBioNP daily and BioS with water change. As tank is new (10/2017) there were some algae loving low nutrients and my NO3 and PO4 were undetectable with salifert but with Hanna fotometer I was able to measure 0,01 PO4.
As some of my corals were too much "neon" or bleached (purple stylopora is pink, cathalaphyilia had purplish "tentacles" and now they are white) I suspected nutrients were too low. After adding 6chromis and foxface to previos 2 bangai adn 2 ocelaris nutrients rised a bit (and foxface cleaned aglaes) so now I measure 0.5-1No3 and still undetectable PO4 (probably 0.01 with hanna, need to check).
What would be your suggestion for bringing back colours to corals?
I belive that feeding fish should be enough and that I'll simply add No3 and P04 if I buy some food like aminoacids or energy supplement. Wouldn't it be simplier (and in the end more cost effective) if I simply lower the filtration with reducing Phosphate minus or so? (currently using 2dcl on 400l tank)
It seems to me that people go in circle with lowering nutrients with chemicals and than raising them with "special" food for corals with end effect in needing more filtration to compete with added food Or I'm totally wrong ?