I recommend you take a water sample to a local fish store to double check your results. The comment is not meant to doubt your competency, but standard practice for all laboratories is to double check their own results against another standard.
I point this out because the EPA standard for maximum allowable nitrate is 20 mg/L (20 ppm). Levels higher than this can be toxic to infants, so probably not great for adults either. The fact that you're getting high results out of the tap water and positive results coming out of R/O suggest something is askew. In a properly functioning R/O system, there should never be more than trace nitrate readings. Also, a TDS reading is not going impact nitrate levels.