I'm interested in the chemistry here. When the iron ion is removed, what replaces it?
With ascorbic acid and sequestrants (probably based on EDTA chemistry) these are simply chelating agents, the Fe2+ or Fe3+ ion has a greater affinity for the chelating agent than, say, the wall of your pool. The iron never leaves the water, it's just bound up in it.
With Fe ion removal, either both the ion and counter ion are removed (perhaps by precipitation of an oxide) or else some other ion is substituted into the water. For example, when your water softener removes Ca from your water, those Ca ions are replaced by sodium.
So where does the iron go?
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