From "How to Talk Border Patrol" - an unpublished manuscript by J. Byrns Long
tank - This word had more than one meaning but in general usage was short for stock tank, a man-made water-filled depression intended to provide water for livestock. In the rest of the English-speaking world, this feature would have been known as a pond, but for unknown reasons, pond was never used for this purpose in Texas. To complicate matters further, there were also numerous oil tanks related to the petroleum industry.