Here's what you do. I had this issue before. SAME thing
What Cna I Use to Absorb Nitrates?
Your nitrates may actually be greater than 80 ppm
. I conducted a dillutional linearity test where you take multiple samples and dillute them with top off water...fresh RO
. You take your test sample and split it between two tubes to make a 50% tank/50% RO
sample for testing. Split again and you have 25/75 and so on. do the calc and you have your reading. Once an API kit gets to 80ppm it gets unreliable.
Ultimately, what worked was doing big water changes every few days. If you can do 40-50% it will speed things up. 10% PWC
's just take longer.
It will come down, it has to. But like other say, think about what's contributing.