Ozone And Ultraviolet Irradiation

Ozone and/or ultraviolet (UV) irradiation may be used to remove pathogens from water containing fish, thus reducing the likelihood of disease transmission between fish or individual aquariums.


As mentioned below, the use of ozone and UV irradiation can bring a number of other benefits, too. However, neither method should be looked upon as a replacement for additional disease treatments, since both only act against pathogens which are in the water, and not usually against those which are attached to the fish.


Ozone (O3) is an unstable form of oxygen (O2) with powerful oxidizing properties. As a result, it can not only bring about a fall in the number of free-swimming microbial organisms, especially viruses and bacteria, but also a decrease in the levels of organic materials that would otherwise be difficult to remove by normal filtration methods. Using ozone can eliminate the "yellowing" of old aquarium water, for example, and also improve the efficiency of protein skimmers in marine aquariums.


Ozone generators for aquarium use are available from specialist aquatic shops. They are usually the type that produce ozone by electrical means (by passing air through an electrical discharge), and are available in a range of sizes to produce different amounts of ozone. The ozone is usually paased into the water via an airstone. Because of its potentially toxic effects on fish, aquatic plants and invertebrates, ozone is usually employed in a seperate ozonation tank or chamber into which water is pumped for treatment, perhaps as part of a total, out-of-tank filter system, or in place of air in a protein skimmer.


Vigorous aeration or passing over activated carbon can eliminate ozone from treated water, before its return. It is difficult to measure ozone concentration in water. As a rough guide only, beneficial effects should be achieved when ozone is applied at a rate of 0.25-1mg per hour per 10 litres of water to be treated. (Estimate this by comparing the output rating of the ozone generator to the volume of your tank). However, the balance between providing sufficient ozone to be beneficial, yet not so much that it is toxic to the animals and plants in the system, will depend on a number of factors, including the precise method of ozone introduction and the ozone demand of the water caused by dissolved and particulate organic matter.


Clearly, more ozone can be safely applied when a system is relatively densely stocked with large amounts of organic matter present, as compared to a newly set-up quarantine system containing only one or two fish. It is known that continuous exposure to ozone, even at low levels, may be toxic to aquatic animals and plants. However, fish may be able to tolerate higher levels for relatively short periods. As a result, it may be best to use ozone in connection with a home aquarium for just a few hours each day, and in response to particular problems, rather than continuously. If at any  time the fish appear distressed, irritated and/or suffering from gill problems, and particularly if there appears to be a build-up in the characteristic bleachy smell of ozone, discontinue ozonation and seek advice.

Caution - Ozone

https://www.realaquatics.co.uk/media/catalog/product/t/i/tick4.jpg Ozone can be toxic to humans. Its characteristic smell can be detected at 0.02-0.05mg/litre, the upper level of which may be too high for long-term human exposure. As a result, always use ozone in a well-ventilated area, and investigate any build-up in the smell of ozone, after first turning off the ozone generator.

https://www.realaquatics.co.uk/media/catalog/product/t/i/tick4.jpg Even very low ozone concentrations can be toxic to fish, aquatic plants and invertebrates, including the helpful bacteria in a biological filter.

https://www.realaquatics.co.uk/media/catalog/product/t/i/tick4.jpg Do not use ozone with rubber tubing, which it attacks and destroys, and ozone may also form toxic byproducts in sea water under some circumstances.

https://www.realaquatics.co.uk/media/catalog/product/t/i/tick4.jpg Ozone may reduce the effectiveness of some disease treatments.
For optimal effects, air is best dried before delivery to many of the availabe ozone generators.


Ultraviolet Sterilization

This is another useful technique for controlling the free-swimming stages of a number of important microbial fish pathogens. In terms of its wavelength, ultraviolet (UV) falls somewhere between visible light and X-rays. UV rays generally kill microorganisms by attacking their cellular contents. Compact UV sterilizing units are available from specialist aquarium shops, and in a range of sizes to treat aquariums and ponds of particular volumes at a given flow rate. They are usually rated on the assumption that the volume of the pond or aquarium passes through the sterilizer two to four times per hour, and are primarily active against viruses, bacteria, fungus and small protozoans. Larger organisms, such as flukes and crustaceans, need considerably higher dose rates of UV irradiation and hence UV is seldom used to combat these organisms. Some success has also been achieved in using higher turnover UV sterilization units to control "green water" algal problems in garden ponds. Such units are of different construction and generally operate with a higher flow rate than those used in disease control, and may not be so effective against fish pathogens.

Caution - UV Sterilisers

https://www.realaquatics.co.uk/media/catalog/product/t/i/tick4.jpg The tubes of an aquarium UV sterilizer produce rays which can be dangerous to the human eye. Do not look directly at an unprotected tube that is switched on.

https://www.realaquatics.co.uk/media/catalog/product/t/i/tick4.jpg The effectiveness of a UV sterilizer will depend on the amount of colour and turbidity in the water. Pre-filtering or pre-treatment with efficient mechanical filtration, activated carbon or protein skimming is usually beneficial for disease control.
UV tubes have a limited life; renew them every 6-12 months.

https://www.realaquatics.co.uk/media/catalog/product/t/i/tick4.jpg Passing water through a UV sterilizer at a flow rate that is significantly above or below the recommended level, or attempting to use a unit in an aquarium or pond that is larger than recommended, will bring about a loss in effectiveness of the unit.



Photo credit: Freepik