Abstract: A system and related method of filling tanks is provided so that tanks do not attain an overfill condition and/or so the tanks are filled to a maximum specified capacity. The system includes an adjustable float switch having a float configured to float in liquid that is filled into a trailer tanker, a mix tank, a frac tank, a storage tank and a variety of other tanks. The float can be set so that when the liquid attains a maximum level and associated volume, the float switch opens so that pressurized air can be communicated from it to a pneumatically actuated tanker valve disposed in a supply line. This shuts off the flow of liquid through the supply line and into the tank. The system can be operable in a various modes, such as a filling mode, a filled mode, a manual emergency shutoff mode and/or a manual reset mode.