Yes, that is VERY high. That would be contributing to all the problems you have been having. Start off with some large twice (or even three time) daily water changes to help bring it down. Vacuum the gravel well also. Don't mess with the filter media while this is happening because the vigorous vacuuming might cause a slight mini cycle and you want to preserve as much bacteria as you can. The goal is to keep nitrate under 20ppm with weekly water changes. If you can't do that you need to increase the water changes. Can you also test your tap water for nitrates? When you fin one you should look at getting a liquid test. And an ammonia test.