Whats you typical water change schedule? How much, how often?
Are you overfeeding? Once a day no more than is eaten in 3 minutes.
Is there any nitrate in your tap water? If so, how much?
Nitrate can only be reduced by reducing ammonia, changing contaminated water for cleaner water, and having live plants. There are also chemical media you can put in your tap water that absorbs nitrate.
Are you seeing any issues with your betta?
While lower nitrate is better, fish are a lot more tolerant to high nitrate than they are to high ammonia and nitrate. People regularly sucessfully keep fish in high nitrate conditions. If you are taking reasonable measures to keep nitrate low, but your fish are fine, is it a problem?