Going to need quite a bit more info to help. Heavy stocking, heavy feeding, any consumers in the tank like macro or corals how many, tank size? That is common with nitrate removal via water change small water changes dont move the needle much its not a 1:1
And export besides water changes.
I.E. Filters, skimmers, ect.
okay tank is a 60 gallon cube, it has a Fiji cube over flow glued to the back of it, which I run filter floss in, I do have a reef octo hang on the back skimmer that I have set to the wettest skim I could get it to. 4 fish in the tank, clown, 6 line wrasse a gramma and a scopas tang. I don’t feed much. Maybe go through a cube of frozen mysis every 3-4 days. Spread out across the few days of course not a whole cube at one go. Far as coral go, I have several frags but there far from grown out so I have to imagine there not big consumers of anything.