If there is rust in your water supply how would you clean it to fill a small rectangular plastic pool?

Rust in your water is caused by iron in your water supply. You can get a water softner and use the softened water to fill your pool, or you can fill your pool with the rusty water and use a scale and metal out chemical that you could purchase at any local pool dealer. If you choose to purchase the scale and metal out I would recommend taking a sample of your water so that they can test to find out the level of iron in your water. They then could instruct you on how much of the product you should use to clear your water up.