How are you testing the water? Did you test your tap or source water? It could be that your source water has high nitrite and nitrate in it? You can do things like put plants in your tank, or use RO water, but first try to do more frequent water changes and test source water.
Did you cycle your tank? I recommend lots of water changes with the optional addition of prime in an emergency, it can temporarily bind excess ammonia, nitrites, and nitrates until you can do a water change.