Personally I wouldn't add anything "chemical" to the tank. Do a couple of waterchanges, if the fish are looking better, maybe do a small waterchange like 10-15%. Do another one tomorrow and the next day. Smaller waterchanges might be better at this point.
As for the carbon, I would imagine whatever nasty was in the tank has likely disappeared or is not as "strong" as it was. Waterchanges should help.
Whatever issue that went on in your tank may not be reflected in any kind of test readings you can measure. However, something is in the water as evidenced by the fish issues. I would just do a couple of waterchanges and see how it goes.
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