The first thing to say is that our nitrate testers aren't the most accurate. You need expensive lab equipment to get really accurate results. And if you are using a liquid reagent tester, one of the bottles (varies between makes) needs to be shaken extremely well or the results will be even more inaccurate.
But having said that, what is the nitrate level in your tap water? The highest allowed level in drinking water in the UK is 50ppm. If your tester shows nitrate above that I'd suspect either something wrong with the tester or your technique.
If your tap nitrate is below 50ppm, something is making your tank nitrate high. The usual suspects are too many fish; overfeeding and not enough/big enough water changes. The usual guideline is that tank water should never be allowed to get to the tap level plus 20.
Have you entered your tank in the Community Creator? What stocking level does it say you have? If it is over 100%, you need to remove some fish.
How much do you feed? Is there ever uneaten food on the bottom of the tank?
How often do you normally do a water change, and what percentage do you change?
If the tap level is well below 50ppm, I would do another very big water change and test again after 30mins (to allow the new water to mix thoroughly). If it's still higher than the tap level, do another large water change next day and check again after half an hour. Once you get the tank nitrate as low as the tap nitrate, keep it there!
If you have tap nitrate at or above the legal limit, you may need to start using RO water to get it down. There is evidence coming out that tank nitrates should never exceed 40ppm, with sensitive fish needing under 20ppm.