Just to share...
Had nitrate issue once. Done what many suggested, 10% daily water change or 20%-30% weekly water change, while reduced feeding at the same time. Although fed my fish only once a day so to reduce some more meaning once every 2 days? Also done that... [emoji15] still nitrates can never go lower that 20ppm.
Chaeto running and growing and trimming and growing... biosphere added, biopellets running also, micro algae too... still not getting there.
But no sudden death of fish or LPS. But still.. high nitrates!! What else??
100% water change ah... but only recommended if tank crash or something more critical. So now what?
Remembered this sentence somewhere I read.... "no tank is the same.." hmmm... well... time to follow my instinct. 100% wc ahh... (not 100% really.. left just enough water for the fish to breathe).
Crossing my fingers.... praying... and more crossing and praying... thank God!!! 3 months later... all livestock survived and guess what? Nitrates FINALLY below 20ppm... its 15ppm... that is a success to me.
Then continue with weekly wc... and now nitrates is below 10ppm.
Again I repeat.. no 2 tanks are alike. So take any advice with many pinches of salt. And don't hold anyone giving the advice responsible if anything bad should happen. At the end of the day, the dicision is after all the tank's owner.
Hope you find a root cause to your problem. I did not find mine, just decided to do the big water change after all fails.