A case of negligence has been registered and the owner of the under-construction house is being questioned, they added.
Arpita (6), a kindergarten student, and her elder sister had gone out to buy some household items.
It appears that on the way, Arpita went to the under-construction house, where she accidentally slipped and fell into the tank, police said, adding that the tank did not have a barrier around it.
On returning home, the elder sister told the parents that Arpita was missing.
The parents launched a search for the girl and found her body floating in the tank, police said.