Ozone vs. Ultraviolet: Comparing Two Common Sterilization Techniques

Author: Valerio Zupo

Have you ever wondered if you need a sterilizer on your tank and, if so, which one would be best? An aquarium scientist explains the reasons for sterilization and how ozone and UV differ.

Natural Sterilizers

There are two very powerful natural sterilizers, ozone and ultraviolet (UV) radiation. Ozone is a gas made of three oxygen atoms, whereas the oxygen we breathe is composed of two oxygen atoms. That chemical structure makes ozone highly reactive, which is why it is such a powerful sterilizer.

In aquarium applications, an ozonizer, or ozone generator, can be utilized to help maximize water quality. With this type of system, air is pumped into the ozonizer unit (sometimes after first passing through an air dryer to eliminate humidity) where it is exposed to, in most modern models, a high-voltage electrical discharge (although some models use UV rays), which breaks apart the oxygen molecules. As the molecules recombine, some ozone is formed. The gas then enters a protein skimmer chamber or ozone reactor where it mixes with aquarium water. The outflow from the chamber is then filtered through activated carbon to remove any residual ozone before the water returns to the aquarium.

Ozone is found in the ozonosphere, a high level of the atmosphere. In fact, the filter created by the ozone layer is vital because UV radiation is quite dangerous to all living things, being able to cause mutations in DNA. That is the reason the hole in the ozone layer has become such a popular topic—the reduction in the ozone concentration, especially at the poles (where the ozone is thinner), increases the UV radiation in the biosphere, resulting in damage to, among other things, our skin and coral reefs. The damage caused by UV rays is important to aquarists because they have tremendous power that can be used to destroy unwanted microorganisms.

Overly Energized

Both ozone and UV work based on an excess of energy—in fact, some ozonizers produce the gas by irradiating the air with UV. Ozone is very reactive because it is highly unstable. To return to a lower energy level it must lose an atom of oxygen, which becomes a free radical—something that is able to oxidize almost anything in the natural environment.

UV rays also contain a very high amount of energy, and they can penetrate cells, reaching the DNA where they can cut the genome and undermine its functionality. After a few seconds, an auto-destruction program (known as apoptosis) is normally initiated in the cell. However, at lower exposures, the cell may survive and simply be damaged.

All higher organisms—both plants and animals—are protected from low exposure to UV radiation by their external structures (such as the skin or cortex). Species that develop in areas that are exposed to larger amounts of UV often develop thicker skin, which is able to filter out the radiation before it can do any damage. Conversely, small organisms such as bacteria and protozoans often lack these protective structures and can, therefore, be very sensitive to UV radiation.

UV rays themselves, however, are also quite delicate and can be blocked by filters such as some gases (like ozone), glass, water, plastic, etc.

UV Lamp Basics

Therefore, it makes sense that the bulbs in UV lamps are made of quartz because quartz does not filter out UV rays. Water, however, does filter the rays depending on its depth. For this reason, the space between the quartz bulb and the external glass or plastic tube where the water flows is generally only a few millimeters—a thicker space would reduce the strength of the UV rays and allow bacteria and other microorganisms to survive.

How to Boost Effectiveness

The effectiveness of the UV lamp is also proportional to its power, measured in watts, and the amount of time of the exposure—the higher the wattage and/or the longer the exposure time, the more effective the radiation treatment. Although the speed of the water can be varied, you should take into account the speed of reproduction of some of the microorganisms you want to eradicate. Some can double their population size in as little as 10 minutes in certain conditions! Therefore, you want to maintain a decent flow-through rate inside the lamp, at the same time remembering that if the speed is too fast, the bacteria won’t receive enough radiation to harm them.

Think of it like tanning in the sun. If you live in the northeast United States, you might need to spend at least an hour outside in the spring to get a light tan, but if you go down to the Florida Keys, you might need to be outside only a couple of minutes before obtaining the same tan. The same happens to bacteria (except they are killed, not tanned!).

Finding the Right Ratio

Thus, the power of the UV lamp and the speed of the pump must be in the right ratio with the size of the tank. In general terms, 1 watt for every 20 liters (5 gallons) of water may work well, and a flow-through rate of about two times the volume of the aquarium per hour is appropriate. The suggestions of most manufacturers are generally lower than that, but the results are not always satisfying when the power of the lamp is lower. Remember that a UV bulb should never go over an aquarium directly, as it will kill the beneficial bacteria used in biological filtration and likely harm the other inhabitants.

Do I Need a Sterilizer?

Now that I’ve reviewed the two types of sterilizers and what they do, an important question must be introduced: Do I need a sterilization system?

In nature, of course, there are no sterilizers. Ozone is restricted to high layers of the atmosphere, where life is almost absent, and UV rays are normally filtered before they reach the ground. Since an aquarium should be a good representation of a natural environment, why should we try to sterilize or disinfect it?

Obviously there is no reason to in normal conditions and, in fact, most aquariums work perfectly in the absence of any sterilizing device. Sometimes, the need to sterilize is just born from our anthropocentric view of the world: Let's sterilize the aquarium to keep it like my clean and perfumed flat!

But remember that in most natural environments, the organisms are found in much lower concentrations than they are in our aquariums. Aquariums that are overstocked or receive few water changes are even worse, as the concentration of bacteria in those setups is greatly increased.

Environments with Low Bacteria Levels

It’s also worth taking into account that some tropical fish live in waters with very low densities of bacteria, less than in any temperate basin. Discus, for example, often live in waters that are almost free of bacteria due to a very low pH and the presence of humic substances or to very efficient mechanical filtration. In fact, discus kept in waters containing a high abundance of opportunistic microorganisms can experience disease problems. In such cases, a sterilizer may be very beneficial.


Finally, our aquarium is sometimes invaded by pathogens, and the addition of a sterilizer may be sufficient to solve the problem without introducing any harmful chemicals. The activity of a sterilizer may also help eliminate excess organic matterand microalgae and/or reduce bacterial explosions (milky water). It can increase the oxidation-reduction potential of the water—making it better for some animals and reducing the loss of eggs due to fungi and mycobacteria.

The goal is to avoid the anthropomorphic vision of the world inducing us to “clean the water by disinfection.” Natural waters are perfectly clean if not sterilized.

Choosing a Sterilizer

Pros and Cons of UV

The choice of which sterilizer to use is easily made if you keep in mind what each sterilizer does. If you want to avoid adding toxic compounds to the water but wish to simultaneously lower the abundance of bacteria and other microorganisms, a UV sterilizer is the way to go. The water will contain much less bacteria after the trip through the UV tube, and, after each cycle, it will be purer, but bacteria present in the biological filter, for example, will not be influenced by this treatment. This also helps keep the water almost free of bacteria and microalgae—the water will be crystalline.

UV sterilization does have a minor drawback: It does not kill microorganisms that are scarcely mobile—those attached to the substrates or to plant leaves—because only microorganisms passing through the plastic (or metal) tube will be treated. It is worth noting that metal tubes made of titanium have a stronger sterilizing power than a traditional setup.

Another possible drawback of UV sterilizers comes from their structure. The internal quartz tube, containing the lamp, is fixed to the external chamber by means of an o-ring or other rubber seal. They generally work perfectly for at least a year but may become brittle or break over time. If that happens, when you reassemble the sterilizer after cleaning, some humidity could penetrate and reach the electric areas, producing a short and possibly introducing poisonous chemicals into the water (especially in the case of seawater). This isn’t a problem with high-quality UV units, so do your research before purchase and still always be careful to check the rubber during each cleaning.

UV sterilizers are generally simpler to use than ozonizers because nothing is dissolved in the water, and the only notable danger is the contact of water (especially seawater) with the electrodes due to scarce maintenance.

Pros and Cons of Ozone

The situation is the opposite in the case of ozonizers. They need little maintenance (only needing the air filter cleaned annually), and they are usually sturdy for life. However, ozone gas, which is directly dissolved in the water, is very poisonous for fish, invertebrates, and humans. Therefore, you must be certain you are adding just the right amount into a contact column (not the main aquarium) and the water is filtered before reaching the main aquarium—activated carbon is effective for filtering because large quantities degrade ozone byproducts. Most manufacturers generally suggest adding 0.3 to 0.5 mg of ozone per gallon of water per hour, but I have found that even half that much is sufficient to keep the bacterial concentration in the aquarium very low and oxidize all the organic matter without hampering the power of the biological filter. The problem is that if excess ozone does enter the aquarium, inverts will die first, followed by fish.

Benefit of Ozone

One interesting benefit of adding ozone is that ammonia is immediately transformed into nitrate, which is obviously a desirable outcome. Also, it is believed (but still debated) that the activity of a skimmer in a marine aquarium will be significantly enhanced when used in combination with ozone. Microalgae and cyanobacteria will have a hard life in the presence of ozone.

Measuring Ozone

Knowing that even extremely low doses of ozone can harm fish and invertebrates, it is important to be able to measure how much is in the aquarium. To determine the correct amount, the best method is to measure it indirectly by measuring the oxidation-reduction potential (ORP) of the water. You start by measuring the ORP before ozone is added, then run the water through the sterilizer and measure the ORP again.

Make Sure treated Water Is Safe

The treated water may be considered safe if the difference does not exceed about 30 mV. Remember, however, that these are not absolute measurements. If the water does not contain organic or inorganic compounds able to be oxidized, then the increase measured will be entirely due to free ozone and its byproducts. In that case, even 30 mV of difference could be slightly dangerous. In other cases, when the water is very polluted, the change in the ORP is only due to the oxidation of various pollutants and the water could be safe even though it is showing a large difference.

Aeration Techniques

To figure out your situation, use strong aeration for an extended period. After three or four hours of aeration, you will observe that the ORP will remain constant, meaning that you reached a new baseline for your water. In other words, all the ozone has been expelled, byproducts are probably absent, and the higher ORP is only due to oxidized organic compounds. Luckily this operation needs to performed only once, when adding the new sterilizer.

The simplest ozonizers can be connected to a contact column (e.g., a skimmer) and the water can be filtered through activated carbon afterward to remove any accidental residue of the gas. In this case, the regulation of the ozonizer can be performed according to the suggestions reported above. Other systems, which are more complex, also contain a continuous ORP sensor and automatically regulate the amount of gas to be dissolved, avoiding problems with fish and invertebrates. Always carefully observe your animals during the first phases after the installation of an ozonizer and be ready to stop it if you note any problems.

Final Considerations

When it comes to deciding what is appropriate for your setup, remember that the two systems have different advantages. Consider, for example, their use against specific diseases. Ozone could be deliberately administered at a higher dosage (reaching 35 to 40 mV of difference) to cure some fish diseases. In fact, the dissolved ozone and its oxidized products directly act on the body (epidermis, gills, fins) of fish and can rapidly kill bacteria, fungi, and some protozoans. For example, it can be very effective for stopping an infection of columnaris rotting the fins of fish or black spots on the body of shrimps.

As for any treatments, the subjects must be strictly monitored. The main advantage is that the oxidizing gas exits promptly from the solution and the aquarium immediately returns to its previous shape, without the need for filtering with activated charcoal, changing the water, etc. Ozone is also perfect when you need to sanitize the water for a reproduction or quarantine tank because you can use ozonizers at maximum strength, then aerate overnight and be sure your water is pure and safe.

UV sterilizers are better fitted to freshwater aquaria for continuous use. They do not add anything to the water and kill everything passing through the circulating pump (if the system is used according to the suggestions reported above), but they will not kill parasites attached to the body of animals or contained in the mucus, those adhered to rocks, etc.

In conclusion, both UV sterilizers and ozonizers can be necessary, sometimes indispensable, to solve specific problems, but they should be used only when actually needed, with the right cautions, according to specific techniques.

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