If it cannot be moved
The south is a heavy direction and the tradition does not put water below it. If a sump is fixed here, keep it sealed and well maintained, and place the overhead tank in the south-west or west instead.
What the ದಕ್ಷಿಣ governs
South is Yama’s direction, and it carries weight rather than light. The tradition builds it up — heavy walls, storage, mass — and keeps openings modest.
Where the ಜಲ ಟಂಕೀ 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.