Pressure tanks are usually either a metal or plastic tank with a food-safe (plastic*) bladder. An air pocket that surrounds the bladder is what pressurizes the tank. This pressure gives you the amount you need to get water out of the faucet and/or the refrigerator. Without the pressure tank, you can still get water out of the system, but not in good quantities. You can use a delivery pump to boost the pressure coming out of it to maintain good flow, but it's more costly. Pressure tanks do have an adverse effect on the DI. This will be discussed more in the reefkeeping section related to the Dual Home Reef systems. The bladder will not leak anything back into the water. This is why it is "food-safe".