I have a 500L RO van mount system. My PPM has gone up to 3PPM today and I decided to change the resin for the first time since I've had the system. I gave the RO system a good flush out and then left my tank to fill up again. Ive just tested my water and its 3ppm again?? I still had 1/4 of a tank of water from the last fill up. Do you think that it has mixed together and made it 3ppm. Or could it be something to do with the filters??