What do you put in the pool to eliminate the rust when you fill a 15' x 42 inch pool with city hard water?

Answer

It's very tough to remove rust stains if they are already established. It sounds like you have an aboveground pool, possibly purchased at Canadian Tire. You can use a sequestering agent (such as Aqua Min-Out) to keep the rust-causing minerals in solution (in the water) so that they don't stain the pool. To remove existing stains, you can try dropping some granular PH Minus (PH Down) into the water so that it settles onto the rust spot or spots. Obviously, this will only work on the bottom of the pool and if the water is still. In addition, ensure that you have your water professional tested to determine the Free Chlorine, pH, Total Alkalinity, Calcium Hardness and Stabilizer levels. Poor water balance can cause problems with the liner, and if the alkalinity is low, any metal parts (in the pump, etc.) will rust.

Answer

Try BioGuard's product called Pool Magnet. It is great for removing various metals from the water.

Just a little added detail you can also purchase a filter that screws onto the end of your hose so the water is filtered clean of minerals before it even reaches your pool water.