Ok done testing, for reference only. Tap water TDS = 110 to 120 Nitrite = 0.5 to 1ppm 4 stages filtered DI water TDS = 10+ Nitrite = 0.1ppm Boiled water TDS = 100+ Nitrite = 0.1ppm NTUC pure drinking water TDS = 4 Nitrite = 0ppm Based on the above result, I reckon to get 0ppm Nitrite in water is either I need to add 1 more carbon filter or to get TDS 0.