The Philadelphia Fire Department was called around 9:20 p.m. Monday to the 1300 block of Lombard Street.
Firefighters went door to door Monday night on the second through eighth floors of the 19-story Casa Farnese Apartments, a complex for senior citizens.
Approximately 200 residents were evacuated from their apartments. They were taken to the first floor for their safety.
"Firefighters were knocking on the doors of the upper floors. One of the maintenance people made an announcement that everyone was to evacuate to the community room with no particular information," resident Rita Addessa said.
Crews worked overnight to determine the source of the leak. The results of their investigation were not made public.
The all-clear was given early Tuesday morning and all residents were allowed back to their apartments.
No injuries were reported.