If it cannot be moved
An underground tank in the south-west undermines the corner the tradition most wants solid — this is the placement where a sump is objected to most firmly. The overhead tank, by contrast, belongs here. Where a sump cannot move, keep it structurally reinforced and covered.
What the South-West governs
The south-west — Nairutya — is the earth corner: the heaviest, most grounded part of the plan. It is where the tradition puts what should not move, which is why the master bedroom and the structural mass belong here and water does not.
Where the Water tank 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.