Nitrites in RO? What to do?

Was doing some testing on my RO water and found like 2 to 3 PPM of nitrites! Flushed the membrane, drained the head tank and flushed it. Did some testing after that and back to 0 PPM. I let the system fill the head tank and sit over night. The next morning it tested .5 PPM of Nitrites. That made me think something was going on in the head tank. The tank has been in service for about 4 years. So, I ordered a new head tank. Tank arrived and I flushed every thing again and installed the new tank. While filling the system I tested 0 PPM. Let it sit over night and I got .5 PPM nitrite again. When I isolate the tank and test the water coming right out of the RO I get 0 PPM. This is a 5 stage filter array. All filters about 3 months old. RO membrane about 1.5 years old. I would hate to replace all filters and membrane just to get nitrites again. My tap water tests at 0 PPM. Any ideas or how to eliminate? The RO TDS is around 15. I am using an API freshwater master kit. Nitrite bottle is good until 02/2019. We use this system for drinking and WAS using for my SW tank.