Krishnamurthy (53) and his two sons Kannan (23) and Karthik (21) were cleaning the tank in their apartment when the incident occurred, the police said.
The other three victims - Paramasivam and Suradha were living in the same apartment while Lakshmikanth was a member of the public who offered his help and also lost his life, police told PTI.
The bodies have been taken to the general hospital, Sriperumbudur, for an autopsy, they said.
According to a senior Fire and Rescue Services official who was involved in the rescue operation, Krishnamurthy and his two sons were cleaning the tank and shortly afterwards there was no response from them.
Then, Paramasivam, Suradha and Lakshmikanth went into the tank and they too died after inhaling the poisonous gas, the official said.
A case has been registered and an investigation on.