Ozone also has a dark side. When reacted with seawater, ozone produces a variety of highly oxidized halogens such as BrOH and BrO3-. If the ozone produced oxidants are not largely removed with activated carbon, they may enter the aquarium and be hazards to the most sensitive organisms in the aquarium (which are likely eggs or early stage larvae).
Finally, ozone alters a variety of other inorganic materials in ways that may or may not be important. It alters the aquarium's redox balance, raising the ORP (which may mean as little as altering the ratios of different forms of manganese in solution). It may permit more rapid conversion of ferrous ion to ferric ion, and may increase its bioavailability, but perhaps decrease the lifetime of strongly complexed iron such as EDTA iron. Ozone also oxidizes ammonia to nitrate. While that is likely beneficial, it may alter the relative effectiveness of different nitrogen export pathways (macroalgae vs. denitrification, for example). It may drive the speciation of iodine toward iodate and away from iodide. Is that good or bad? I expect neither, although others have different opinions, but it is a good poster child for the many things that happen in reef aquaria when using ozone that normally take place without any notice or recognition of them by the aquarist.
So with all things considered, is the use of ozone in a reef aquarium worthwhile? Many aquarists answer with a resounding, "Yes!" I'll leave that question unanswered until additional information is detailed in the next two articles discussing what equipment and methods are most useful for applying ozone to aquaria, and reporting on what impact it had in my aquarium.
