Check your tap water for nitrates to see if you get a measurement. If you do water changes regularly, I really don't see why you would get that high of a reading. Perhaps you should do water changes more frequently. If your tap water tests for high nitrates, then I suggest you start using cleaner water...ie. r/o.
I have the opposite problem. My nitrates are at 0. I have to dose nitrates to keep my plants healthy. This only applies to heavily planted aquariums.