Testing RO water with traditional reagent based NO3 kits will generally not be accurate. This is because the reagents used are only capable of working within certain Ph tolerances as they often include weak acids. Ro water has no buffer system as its so pure, as such the reagents used effectively spoil the sample as they will push the Ph outside the resolution of the kit. To work the sample must be able to resist the Ph change of the reagent, if the sample has no buffer the Ph will swing wildly with the addition of any acid or alkaline solution even if it is very weak.

To gain a better idea of what is going on it maybe better to mix in a small sample of Nitrate free aquarium salt in a very clean glass 35g of salt per 995gr of water and test again, be mindful that washing up detergent is high in nitrate and Po4 so make sure the mixing vessel is spotless and well rinsed in RO before you make up a stock salt mix to test. This should provide a better idea of whats going on if the Nitrate kit is accurate.

If you are still reading high nitrates it is best to compare the results with another kit and check the TDS coming out of your RO unit. If you have a high TDS it could be a sign the RO unit is due maintenance or has not been installed correctly.