Took me a bit to figure this one out.  After I tried every product on the market I found out I had bigger problems.  After just a couple washes my clothes were a dingy yellow color.  We decided to get a water softener and what a difference that made!  I don't have to use a ton of soap or whitening products any longer.  If you are having the same problem give a softener a try.  We called RainSoft and in less than a week we were all set up, they even tested our water to confirm my thoughts on the iron http://www.rainsoft.com/home-water-testing.cfm
 (http://www.rainsoft.com/home-water-testing.cfm)  Wish I would have thought about this a year ago!