Ksquared, I can't speak for Colt's tanks, but the ones I've seen were manmade by removing earth on a downhill slope and using the earth to form a dam. I would imagine that water wells are used to supplement rainwater that's captured, but I really can't remember about that -- it's been a long time since I've seen a tank up close.
I can verify that tanks have snakes in them. A friend and I were fishing at a tank on his family's property, and there were a number of water moccasins poking their heads above the surface of the water. We had caught several fish and had them on a stringer in the water, and when it came time to leave, I pulled the stringer out of the water to find that a water moccasin had a fang stuck in the stringer cord. Fortunately the fish were intact and we were able to shake loose the snake. An unnerving end to a fishing trip.
I suspect I didn't answer your tank question very well, as my knowledge of tanks is very limited. I'm sure that Colt can fill you in whatever I missed or was totally wrong about. <g>
As for my picture that you "saw", I wish I could post it as well. I really do need to get someone to show me what's needed to get a photo on a computer file and then get it to appear on a message board.
Justin