It has long been recommended that ozone be used in marine tanks for many reasons including its ability to kill many pathogens and make the water crystal clear. Ozone (O3), which is an unstable form of oxygen, acts by oxidizing with (giving up an oxygen molecule to) anything that it comes in contact with. In laymen's terms, ozone acts as bleach and kills things like bacteria and viruses on contact. Not only does this bleaching action help reduce the pathogen load in a tank, but ozone also interacts with dissolved organics in a tank to make them less interactive so that they can easily be removed by a protein skimmer. This is why when the saltwater hobby was starting out forty years ago, the use of a protein skimmer coupled with an ozonizer was highly recommended. Fortunately we have come a long way since then. Those old ozonizers created ozone via an electric spark, much like lightning does. Newer ozonizers are now in a solid state and produce ozone through corona discharge technology where oxygen molecules pass over a specially designed light source to produce the ozone.

Ozonizers clean the water chemically by having this extra oxygen molecule interact with living material as well as with the by-products of metabolism. The relative cleanliness of the water can be measured by the reduction-oxidation (redox) potential of the water. In most healthy systems, this redox potential is usually between 300 - 350mV without the use of ozone. In order to measure the redox potential a special electronic monitor and probe needs to be used. With ozone, redox potential can easily be raised to 400 - 500mV. This higher level is closer to that seen in the waters surrounding a reef, where levels of dissolved organics are very low. This is why the fish seem to be suspended underwater. The same effect can be achieved in an aquarium with the use of ozone. The ozone removes much of the yellowing compounds that tend to accumulate in a reef tank over time. By intermittently using ozone the water can be kept crystal clear, even more so than can be achieved with the use of carbon.

Ozone does not need to be run constantly, but can be run for short periods of time once a month to keep a tank crystal clear or when the redox potential drops below a certain level. Ozone should also be utilized when a tank is being started and live rock is being cured to remove as much of the dissolved organics as possible. Ozone should also be run constantly when a disease breaks out. Ozone cannot affect the pathogens that are on the fish, but if any free-swimming pathogens come into contact with it in its reaction chamber or in the skimmer they will be killed. Ozone needs to be kept out of the tank as it bleaches any organic matter that it comes into contact with. It should either be bubbled into a protein skimmer or into its own reaction chamber. When it is in use, the water should then flow over carbon to keep any ozone from reaching the tank itself. If ozone does get into the tank it can cause bleaching of the gills of fish, so this needs to be avoided.

As noted above, ozone is highly interactive and as a result it needs to be used with special equipment. Any tubing that it passes through needs to be of silicon, which does not interact with ozone. If regular airline tubing is used, it will become hard and brittle quite quickly when exposed to ozone. Similarly, most plastics will become brittle and crack when exposed to ozone so be careful when choosing a vessel for introducing ozone. Ozone also breaks down quickly when exposed to high humidity so it is best to run the air going into the ozonizer through an air drier. This will dramatically increase the production of ozone.

As noted above, it is not necessary to run ozone constantly in any system. However, having ozone available when necessary is a worthwhile investment, especially when its use makes the tank so clear that the fish seem like they are suspended without any water around them.

Mike Paletta is the author of The New Marine Aquarium and Ultimate Marine Aquariums. He has been in the hobby for over 15 years and has written numerous articles for Aquarium Fish Magazine, Tropical Fish Hobbyist and Aquarium Frontiers.