A fluid immersion cooling system has a fluid tank containing a hydrocarbon dielectric fluid as a coolant fluid. One or more components of an electronic system is immersed in the coolant fluid. A gas cylinder contains a non-flammable, compressed filling gas. The temperature of the coolant fluid is monitored during operation of the electronic system. The filling gas is released from the gas cylinder and into the fluid tank when the temperature of the coolant fluid rises to a trigger temperature that is set based on the flash point of the coolant fluid. The filling gas covers a surface of the coolant fluid to block oxygen from interacting with vapors of the coolant fluid to prevent combustion.