You can use Prime and see if that will reduce the nitrates. Don't disturb the sand or the live rock while doing a water change or just in general. Leave it alone when you do water changes. Its impossible for the nitrates not to decrease after a water change. Do you use any kind of a mechanical filter? If you are using frozen food. Stop using it and see if that makes a difference. Use something like Cyclopeeze when the filter are turned off. Also do you turn off filters while feeding? I turn off all my filters for 2 hours during and after feeding. Get all new filters. That should help.