Good avice so far. If you have measurable nitrates, consider removing the bioballs from your wet-dry if you're using them. Do this slowly over a period of time, and only if you have a sufficient amount of liverock (approx 1lb/gal, but less is needed if you have a sand bed) OR a decent sand bed, like 3-4" of depth of fine grain sand like aragamax or southdown. This will result in a more balanced population of bacteria in areas of varying oxygen content, which should remove your nitrates.