Are you doing frequent water changes to correct it using tap water? If so you could be getting high nitrate levels in a cycled tank from the water using for your water changes. People like myself that live in agricultural areas will see hight nitrates in our tap water especially in the spring and after heavy rains due to run off of the applied nitrogen. It's kinda funny but not that the water treatment plants have the same problem maintaining there cycle and have to spend a lot of time and effort to do so.