What does the water bottle do?
A bottle filled with salt water is supposed to keep the fresh water in the tank from freezing.
I read through dozens of posts in several forums researching this matter. I found only one plausible explanation as to why putting bottles of salt water in a chicken waterer prevents the fresh water in the same tank from freezing. I found no plausible explanation for the same effect in a stock tank.
Important... even then adding a 16 or 20oz bottle of salt water only changes the temperature that the fresh water in the tank freezes by a couple of degrees at most. So, if the outside air temp drops below... say... 25 degrees and stays there a week, the water in the tank will eventually freeze. This means putting small bottles of salt water is only going to help on nights when the temp drops below 32 degrees at night but warms up above 32 degrees during the day. This will delay or even prevent the fresh water in the tank from freezing at night.
The following is by a guy who claims to A) be a chemist B) ran an experiment with two identical chicken waterers, one with salt water bottles inside the tank and one without. Since I don't know this person I recommend taking his theory with a grain of salt... pardon the pun.
The reason a bottle of salt water suspended in a chicken water helps keep it from freezing up is because the salt water in the bottle doesn't freeze at the same temperature as the fresh water in the container. However, it won't help in an open top stock tank. As others have stated, the temperature of the salt water is the same as the fresh water, it just doesn't freeze at 32 F.
As the water in the chicken waterer (a sealed container) begins to freeze it begins to expand. The salt water doesn't and that raises the pressure slightly in the sealed of salt water bottle and the chicken waterer, that slightly lowers the freezing point of the fresh water in the chicken waterer. An open stock tank won't have any increase in pressure in either one.
I know this is more information to say the same thing that has already been said: no adding a salt water bottle to an open stock tank will not help keep it from freezing up.