You know me, take many years to "calibrate " my eyes , i wont use those water test as first option , i go by read the color of the corals , how happy they are, water flow tell you, some, brown, green color tell some, location up or down, etc. In my personal opinion , we have 2 major issue to control.
# 1. .......Detritus.
The faster and safe you exported , the better is ( PO4, NO3 issue)
Constant Alk. also Nitrate.
If some how control the NO3 (constant) Denitrification Sulfur Reactors , works great for me, If some how NO3 is constant to 4 ppm ( in my case) PO4 stay 0.02 to 0.06).
I believe in BIO, healthy water ........show you ...... healthy coral,,,,,,crazy color.
I am so happy , i do NOT use GFO anymore ( list for now).