Only a deep sand bed (generally 4-6" of fine grain sand) is going to reduce nitrates. If your sandbed is relatively shallow, the only thing it's going to do is help convert ammonia to nitrates, which isn't going to help you. If for some other reason you want to add more sand to your display, I'd probably just add dry sand, as there should be plenty of life in a 6 year old sandbed to seed any new sand rather quickly. I'd look at other options to reduce your nitrates, though.