For dealing with iron, cal hypo is much better. The only downside to cal hypo is that you MUST use it the way I instruct you to do, or your will end up with calcium that is too high. Well, I guess the fact that you may not need as much as you'll have to buy is a downside. It will keep till next year, but will be 'stinky' when you open it next spring

This is the ONLY product I've found that you'd want to use:
CCH Calcium Hypochlorite Tablets 50 lb bucket

The Arch cal hypo tablets contain 25% or more magnesium sulfate, and you don't want that in your pool.