In the vastu purush concept, should the NE diagonal be kept free of an underground water tank?
Correct, not on the exact diagonal where the head of Shikhi devta falls. Vedic vastu calculates the Shishu Pishach points, and no shilanyas pit, water pit or any other pit is dug on them; this is taught in the diploma.
Related questions
Why do an underground water tank, drawing room, puja room and conference room suit Shikhi (E1)?
When Adi Prabhu (Shiva) did sadhana on Kailash, the friction produced heat that began to melt the mountain, so water matters most for cooling this place; Shikhi's energy stored with water also spreads through the house and keeps removing about 50% of its vastu doshas. In the drawing room the family sits as if in sadhana and makes its plans and policies, and in a conference room others are convinced with one's ideas and vision and given targets. Worship of Adi Prabhu (Shiva) in a puja room here raises the energy further; an office and a borewell also suit this pad.
Can a bedroom be made at Shikhi (E1), and why should it be white rather than red?
Yes, a bedroom at Shikhi can suit elderly people, and it should be painted white rather than the zone's red colour, as advised for people of kapha nature and for those of pitta nature whose heat rises quickly. Shikhi is highly sensitive and already hot, so red would add heat and open the third eye.
Can remedies done after construction fully activate the 45 devtas?
No. A lot of misleading talk goes around about the 45 devtas. The maximum remedy has to be done when the land is prepared and built on; the other remedies taught for later cannot activate a devta 100% and bring it only to about 30 to 35%.
Do the Vastu Devtas reside in the sky or on the ground?
On the ground. That is why they should be established at the shilanyas itself, and their bhog samagri is offered on the ground at each devta's own place.
Does the coconut kalash for Parjanya work when kept outside a door at E2?
Yes. In a flat seen on a recent visit, whose door is exactly at E2 (Parjanya), the resident keeps a water-filled kalash with a coconut on it outside the door and changes the water daily; the coconut has grown into a large plant, and the resident reported very good results for ten years without needing any planetary remedies. This is exactly the remedy prescribed for Parjanya.
How can one tell whether Shikhi devta is activated?
Three tests. Offer nine aahuti of half a spoon of desi cow ghee with half a spoon of rice over burning Bhimseni kapoor: if the ghee fragrance spreads through the whole house up to the pishach bhoomi, the devta is active. A ghee diya at the spot that burns longer than the same ghee lasts elsewhere also shows activation. Sow wheat in soil in a leaf dona, as in Navratri: jaware about 11 inches long in seven days mean full activation, about 7 inches mean moderate, and then the activation methods should be done.