If your supplier has the same water and says it's fine then you'll probably be ok. <5 is ideal but there are many people with nitrates up to 20. If you keep up with water changes and have good filtration you shouldn't have any issues. I certainly wouldn't want to go any higher.
Using resins can be extra work. I'm unfamiliar with this specific resin but the resin used in RO units to produce DI exhaust pretty quickly, and when they do the water stinks and it can be costly to constantly replace the resin.
With nitrates at 20, I would be more concerned about bioload than the nitrates alone. Water changes and regular maintenance will take care of bioload http://forum.simplydiscus.com/showth...-Are-Important