So is this an in van set up?
If your reading is 000 at the point of exiting the membranes and then showing a higher reading once in the tank. Have you checked your hoses for blockage of some type of contaminate?
Its very unlikely that something in the tank or due to the tank is causing you this problem, just by the volume of water in the tank.
I would check all your connections and hoses from membranes onwards