Kinda depends on the scale of a tank build I think... If someone starts up a 20G with HOB filter, no canopy, etc (small build) then I'd say its whenever you get water into it.
On the other hand, if someone starts a large scale build (elaborate stand/canopy, custom built tank, big time skimmer, etc) once they start building and have devoted alot of time/money into it, I'd say its born then.
