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I don't recall ever reading about this particular situation nor does it seem that GAC would strip out inorganic nutrients that algae require for growth. The switch to a different brand of GAC seems like a coincidence.
Running 300-400 gallons per hour through a small amount of GAC might not be allowing enough time for impurities to be in contact with GAC and to be completely removed. For your set up, it seems you cannot be stripping your water to an ultra high purity. Still looks like a coincidence.
Other aquarists have reported their Chaeto dying (melting away) mysteriously. Without more information, we cannot eliminate the possibility that your Chaeto was dying before the GAC brand switch.
As for dosing iron, that can help algae (good and bad) growth in some situations but you are pretty much on your own on how much to dose and how often. An article in Advanced Aquarist summarizes the information on this subject.
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