According to the police, Raj Kumar, Rinku and Nanhe had gone inside the tank to rescue Binesh Kumar.
Kumar later regained his consciousness during treatment at a hospital.
The incident took place at around 10 am in JNJ Electronics Ltd. in Sector 37 Industrial Area when maintenance staffers were removing sludge from the tank.
The company had announced holiday for employees on account of Dussehra today and only the maintenance staffers were called for cleaning the tank.
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Kumar did not know what happened after he became unconscious, but his three colleagues waiting on the ground quickly went down for his rescue.
They too lost consciousness after inhaling poisonous gas.
"All four maintenance staffers remained stuck inside the tank for over half-an-hour till fire fighters rescued them," said Vijay, another staffer.
Gurgaon Police PRO Ravinder Kumar said that an FIR has been registered against company officials for causing death by negligence. No one has arrested so far, Kumar added.