Best placed in the ઈશાન
The north-east is the water corner and the classical place for an underground tank, a sump or a borewell. The one caution is that this applies to water *below* ground — an overhead tank in the same corner is the opposite instruction.
What the ઈશાન governs
The north-east — Ishanya — is the most protected corner in the whole mandala. It carries water and belongs to Shiva, and every school agrees on the same instruction: keep it light, clean, open and low. Almost every serious Vastu fault is some version of loading this corner.
Where the જલ ટંકી 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.