What kind of tank - SW
How high is high?? What size tank & occupant?
, the way to reduce nitrate is by water changes. A nitrate that is not coming down usu. means the tank's carrying capacity is exceeded. You can temporize by doing bigger & more frequent pwc
's, but eventually, a bigger tank is the answer. <Before we go down that road, do check that your nitrate test is accurate! The kits are pretty notorious for giving false readings, esp. if you don't shake up the reagents exactly as instructed. Also, test your tap water to make sure there isn't nitrate there to start.>
, there are de-nitrogenator set-ups. And nitrate levels in a planted tank is a totally different beast.