Having the same issue and looking for solutions myself. I havent tested in years for either nitrate or phosphate as life has been a bit problematic since 2018.
Getting back into things and added a few coral pieces about a month ago along with a 10% WC and the start of adding aminos. I should also mention my salt for the WC was from a brand new bucket.
I'm not sure what set things off however my beloved Goni took a turn for the worse and is still struggling so I'm searching for answers myself. Funny thing is two of the new corals are a Goni and Alveopora and they are both doing well.
Tested phosphate and nitrate yesterday with brand new reagents for both kits (red Sea nitrate pro and Hanna ultra low phosphourus) got 0 on the nitrate and .012 for phosphate. Other levels are Alk @ 7.3, calcium is 420 and mag is 1440.
My system is 4 years old, not a lot of LR but I do have a Marine Pure block in the sump so I'll more than likely remove it and see what impact it may have before I consider dosing any nitrate