Water Expansion Tank precharged at 25psi. House pressure at 70
If it is a "bladder tank" that is, it has a balloon inside to contain the air needed for pressure, do not add any more air to it. when the tank fills with water, the balloon will reduce in size since it is not at the same pressure as the water entering the tank. After the same amount of air is compressed in the balloon/bladder, the pressure will increase to match the incoming pressure. This aids in the cycling of the water pump. Either too much or too little air will increase the amount of times the pump runs to maintain pressure, making the system work harder.