I don't want to complicate the discussion, but I'm confused by your original post.
Specifically, I'm confused if your concerns are with NITRITE (NO2) or NIRATE (NO3)?
Based on the numbers you quoted in your original posting, I'm not surprised that your NO2 levels are only 5ppm ... they follow very closely behind the drop in Ammonia (NH4).
The fact that your NO3 levels are still high, several folks have already commented on this ... NO3 will not drop unless you remove it (either with a water change, or some other nutrient export method such as a refugium or carbon dosing).
If your concerns are over NO2, stop worrying. Unlike freshwater aquariums where Nitrite can kill your fish, Nitrite in a saltwater aquarium are much less of a concern. In fact, many people don't even measure NO2. For more context, read this You'll need to scroll through he article just past half-way to get to the discussion on Nitrite):
http://reefkeeping.com/issues/2004-05/rhf/
If your concerns are with NO3, there are three basic steps here:
1) Stop adding nitrates from your water source. An RODI unit will go a long way here (if you don't like the 5-stage BRS unit, check out SpectraPure's MaxCap system)
2) Increase water changes until levels come down
3) Implement a means for exporting Nitrates. If you don't want to add a refugium, you can consider adding MacroAlgae directly to your main display tank, or try carbon dosing to increase the effectiveness of your skimmer. My preferred approach is Macroalgae in the display ... read more here:
An online guide to the identification and care of marine plants and macroalgae in the aquarium.
www.marineplantbook.com
Hope these help!
Don't hesitate to ask if any of this doesn't make sense!