I really think you should try a new test kit and your nitrate is not as high as you think. I went through the nitrate problem and ended up calling API. They explained to me that if you use the bottles without shaking you are using the liquid without any of the active ingredient that gets stuck at the bottom of the bottle. Then when you finally do shake it like you're supposed to, the ratio of active to inactive has changed and you will be getting way too much of the chemical that reacts with the nitrate and give you false high readings.
He said it happens all the time to their customers. You have to get a new one and shake it every time.
As a side note, I really did have high nitrates because of starting my tank with bad tap water. After several huge water changes it was still too high. I tried Instant Ocean Natural nitrate Reducer and it brought it down to zero after several weeks and stayed at zero without adding any more. Tons easier than all the water changes, of course I still change 10% every two weeks. The owner of a trusted LFS
said this is using a theory similar to vodka dosing but safer. I know there are many purists on this site who advocate only natural things and water changes.