I don't know where you read that but it is unlikely to be true. The active chlorine level in the water with an FC that is 5% of the CYA level is less than then equivalent of 0.05 ppm FC with no CYA so 20 times lower than normally found in tap water. The active chlorine level is proportional to the FC/CYA ratio, not to FC alone. A high FC, by itself, doesn't mean anything except how much chlorine is in reserve -- it does not determine chlorine's strength (reaction rate). The chlorine bound to CYA reacts at least 100 times more slowly than unbound active chlorine (i.e. hypochlorous acid).
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