If it cannot be moved
The north-west is the corner of movement, and the tradition does not want stored water settling there. An overhead tank is tolerated; a sump is usually shifted to the north-east if the plot allows.
What the বাযব্য governs
The north-west — Vayavya — is the air corner: movement, circulation, things that come and go. Guests, storage in transit and utilities sit here easily, and nothing meant to be permanent does.
Where the জল টংকী belongs
Water storage is the clearest case of the mandala’s logic: underground water goes to the water corner, overhead tanks to the heavy one. Getting this backwards — a tank overhead in the north-east, a sump in the south-west — is among the faults the tradition treats most seriously.