A security device protects the vulnerable portions or accessories of a bulk storage tank or container against unauthorized access. The device includes a housing that covers the vulnerable portions of the tank, the housing having two or more lockable doors that selectively permit access to the tank or container via two or more respective apertures. A first door is openable after unlocking a readily-accessible lock, while the second movable door is openable after an elongate lock member that is positioned internally within the housing is moved to an unlocking position. The elongate lock member can be moved to its unlocking position after the first door is opened. Certain tank operations, such as dispensing fuel, may be carried out after unlocking and opening a single door, while other operations, such as refilling the tank, may be accomplished after opening a second door once the first door has been opened.