A safety device for a fluid storage tank having a first port for filling the tank with fuel oil and a second port for allowing airflow therethrough is disclosed. The safety device includes a tubular body having a first open end and a second open end connectable to the second port, and a flow check valve contained within the tubular body and configured to remain open when air flows through the safety device and configured to close based on a predetermined fluid force of the fuel oil through the safety device. Related systems and methods are also disclosed.