Wikipedia
A stock tank is used to provide drinking water for animals such as cattle or horses. Stock tanks can range in size from 100 L to over 5500 L (30 to 1500 gallons) and typically are made of galvanized steel. These tanks are filled either by a pump, windpump, creek, spring, or even rely on runoff water from rain or melting snow.
There are thousands of water ponds in Texas rangelands that locals call a " tank". When the weather is hot some people use them for swimming.
Usage examples of "stock tank".
I remembered all too well the time when all the little monsters had popped up in the middle of the Huntingdons stock tank.
While the workmen wrestled the stock tank off the pickup, the scientists began staking out positions in the sand, setting up chairs and umbrellas and laying out coolers.
He flew over the Tuttle Ranch headquarters, the big tile-roofed ranch house, and the row of mobile homes where the hired hands lived, the barns, the stables, the horse pasture, the stock tank with its connected windmill.
The sun was just easing down toward the Western horizon, the builbats were dipping toward the stone stock tank that Deets and the Captain had built long ago.
Moving through a farming, ranching area dotted with fields, side roads, and an occasional stock tank along the former floodplain of the river, they approached the southernmost all-New Mexico community in the area, Santa Teresa.
Couple of dozen head died from drinking at the stock tank we filled from it.