Tank fittings maintain an adequate air cushion to provide sufficient pressurization against fluctuations of water volume. Since air can be absorbed in water, the tank fitting maintains collected air within the expansion / compression tank. All free and entrained air collected by an air separator or other air collection device is routed through the tank fitting, allowing air to pass freely into the tank. The fitting restricts the flow of water between the tank and the system, resulting in lower tank temperature, minimizing air absorption. Under normal operating conditions, the tank fitting readily allows air to enter the tank.