Sounds like you have a drinking water system. Are you using the pressure tank to store RO/DI water or just RO water or are you using it at all? The post carbon GAC filter is for RO drinking water taste and odor, often RO has a rubbery taste so the carbon takes care of that. But, you do not want to use that GAC for your RO/DI water since it actually adds TDS back as the carbon degrades and turns to dust. Have you read any of my threads where I talk about how to install a check valve between the RO and the DI so you can still use the pressure tank and GAC for RO only drinking water and other uses and make DI water directly from the RO membrane without the pressure tank and its stored up TDS creep? If you are using the pressure tank and want RO only water too then follow this diagram and install the check valve between the RO and DI to isolate the DI from the tank: http://spectrapure.com/huds/4-STAGE-DWK-RODI-NAG.pdf If you do not want or need any RO only water then eliminate the pressure tank and the GAC carbon completely to elimiate the TDS creep issues associated with pressure tanks and short filter runs. You never ever want to store RO/DI in a pressure or bladder tank, it is agressive and starts eating the metal and rubber bladder.