Stan,
If you have your water pump turned on and the city water turned on at the same time, the source of the water that you are using will depend upon the pressure and flow rate of the city water supply. If the pressure in your water system falls below the turn-on point for the pump, you will be using a mixture of both. But if the city water supply is able to maintain pressure above the turn-on point for the pump, you will only be using city water. If you only want to use tank water, turn off the city water supply.
Gerald