The girl, who lived in Madipur, had come to her grandmother's house.
The incident happened around 2.30 PM when the girl was playing outside her grandmother's house in Sangam Vihar's JJ Colony, police said.
The girl later disappeared and her family made a PCR call.
In the meantime, a neighbour spotted the girl lying in the water tank of a nearby house. She was pulled out and taken to a hospital, where doctors declared her dead, they said.
A case of causing death due to negligence has been registered against unknown persons at Sangam Vihar police station, police said.
Disclaimer: No Business Standard Journalist was involved in creation of this content