Not cleaning your filter for months would certainly cause a high number, it's not like freshwater! If your using a canister on a salt tank you need to be cleaning it at least once a week to avoid waste build up. As mentioned before, your beneficial bacteria live In your rocks and the canister is just holding waste and putting it back in your system. I can remember if you use a protein skimmer? If so I would just remove the canister and if not, I would get one and remove the canister
you will still need to do weekly water changes which I assume you do already? Def do a couple of big ones back to back now, if that is a true nitrate reading it's pretty high!