Well, if you want to spend the money, you could get an iron filter (do an internet search). I think they're largely for treating drinking water, but I'm sure it could be used on fish water, as well. I'm also under the impression that a lot of iron in water isn't actually dissolved in, but is rather in particles that you could probably filter out with a good fine mechanical filter, but that really depends on what sort of filtration you are using. I think activated charcoal also claims to help remove iron, but I don't know how true that is.
Hope I've helped a little. Good luck!