Nitrates huh. How many fish are in the tank and do you have any plants to help like hornwort or java moss? If you just can't get them down do what I did when I was having nitrate problems in my 30T, 50% daily water changes for a week and see if that helps.I would do that totally but the 40B they are in is having nitrate issues, I can't get them pulled down and I'm not sure why. That tanks on hold for additions at the moment.