..............to remove the nitrates:
step up your water changes--at least every two weeks if not every week for awhile until you get no detectable readings for nitrates
In the long run you might consider
adding a refugium with chaeto algae--great reducers of nitrates and phosphates plus add extra copopods to your main tank for food.
running carbon and phosban in phosban reactors---the picture below illustrates this--cost about 70 dollars for the setup
