You should try to reduce your nitrates to 0. Good first step. Some more information about your tank might provide a more solid answer for you. Tank size/livestock?
The addition of a protein skimmer might help some, as it will remove waste from the water column. Keep in mind this will not directly reduce nitrates. Less waste = less breakdown = (in the end) less nitrates. Kinda a concept that does and doesn't make sense. I just stopped thinking about it too hard and knew it was good for a reef tank.
Another option might be a refugium setup or an algae turf scrubber. Both are the same basic idea. Growing algae outside of your display tank. Algae consumes nitrates as it grows. This in turn will require you to trim algae, as once it becomes so large it will stop consuming nitrates or die, falling back into the water and releasing the nitrates that it absorbed.