Randy Farley-Holms wrote in his article:
Does GAC or any other of these methods work well enough for reef aquarists to use ozone without undesirable side effects? The answer likely depends on the care which is used in the GAC treatment, and the aquarist's tolerance for OPOs to pass into the aquarium. The answer is likely not well enough when using the highest doses typically used by aquarists and the lowest tolerance for OPOs (that is, the lowest levels likely to cause ANY undesirable effects). Because it is not easy for most aquarists to measure low concentrations of OPOs, the most prudent course of action (aside from not using ozone) is to pass the ozonated aquarium water over as much GAC as possible before letting it re-enter the aquarium.