UV Sterilizers and Their Use in Aquaria

Written by


Thinking about purchasing a UV Sterilizer? Navigating equipment choices, set up and problems.

This article was contributed by Aquarium Advice member Timbo2

Thinking about purchasing a UV Sterilizer? You may ask yourself, do I really need one? How will I know it is working or not? What size should I buy? These are all valid questions and in the next few paragraphs, I will try to explain UV Sterilizers and how you can implement them into your system, to help maintain a virtually disease and parasite free habitat for your aquatic friends.

UV Sterilizers have been in use for many years and has provided the owners with a level of confidence that the chances of fish contracting a parasite or disease are lessened to a great degree. This of course is based upon implementation, strength, and contact time (flow). The way it works is, water from the tank flows through a chamber around an ultra violet light source, and then flows back into the tank. The return water is “reduced” of the microorganisms that pass through it.

Are UV Sterilizers needed to keep a system free of disease and parasites? This is a tricky question and a controversial one. Some say yes, and some say no. The UV light is an optional piece of equipment for preventative use, and should be used as that. Many make the mistake of purchasing one and then find out that parasites are still in the aquarium. I would suggest that the use of a UV Sterilizer should be thought through very seriously. If your tanks is doing fine, and you don’t have constant outbreaks of disease and parasites, as well as algal blooms, then the money you would spend on it could be used for a more immediate purchase. However, if you have problems with the fore mentioned, then a UV could help reduce the amounts of free radical microorganism’s that are causing problems. Keep in mind that this is a lengthy process and you will not see results for at least a week. If you need to get rid of an immediate illness, quarantine and treatment is still the best option.

What size UV Sterilizer Should I get? UV Sterilizers are created in many different sizes from 8 watts to 240 watt for the aquarist, and even higher wattages for larger applications. What wattage to go with is decided by the size of the aquarium you plan to use it on. The unofficial rule of thumb in determining the wattage is 10 watts per 75 gallons water volume for saltwater application and 10 watts for every 150 gallons for freshwater closed systems. In pond application this is greatly increased to 10 watts per 1000 gallons, due to the suns natural capability of producing UV rays. It never hurts to get the next size up from the minimum to allow for bio-load, particles, and turbidity of the water. These factors affect the efficacy of the unit.

How fast should the flow be through the UV Sterilizer? This is the most important part of the setup next to wattage that matters. Flow through the UV Sterilizer is what determines the kill ratio of the microorganisms. Too fast, and they slip right through. Too slow, and the water can heat up the aquarium more than intended and lessen the life of the bulb. When choosing the speed for flow, I have found it best to go with the manufacturers minimum flow rating and increase the flow 50gph. I have found this to be most effective. If you find that it is causing a heat problem, you can increase the flow 50 GPH or so until the heat is not an issue. The key is to stay under the maximum effective flow rate by 50gph. This will present the best kill ratio.

Note: Some manufacturers make submersible pumps and power heads with variable flow selectors. I would recommend getting one of these to be able to adjust the flow. You will want to get the pump rated for just under the maximum flow suggested for the UV, i.e. if 500gph is the maximum flow rating from the manufacturer, you may want to get a 450gph pump. This will give you the ability to adjust the flow accordingly if heat becomes an issue.

How do I attach it to my system? Most of the UV Sterilizers are inline units and require installation on the return line of your pump to the main tank and some are designed to hang on the aquarium like a hang on back power filter. However, with the many designs and customers setups, this question is ultimately between you and the manufacturers instructions based on the unit that is purchased. I would suggest reading the instructions to the best of your ability and if problems arise, see if the manufacturer offers a help line or check out the best online sources such as AquariumAdvice.com.
Note: It would be a good idea to use inline shutoff valves to aid in performing maintenance. This will allow you to take the UV out to clean it.

How do I know it is working or not? For most aquarium owners, this is a question that is asked often. The only way to answer this, is, has there been an outbreak of illness or disease since the addition of the UV Sterilizer? If no, then possibly it is working fine. If the answer is yes, then there may be some things to check like bulb life, cleanliness, and flow rate of the UV Sterilizer. All of these things affect its efficacy.

What does it look like inside? How much maintenance is involved? The inside of the UV Sterilizer can differ greatly per manufacturer, but all work in the same way. Most of them have a canister type outside with a screw on end cap. In this end cap, there is a hole that accepts a quartz sleeve. This sleeve is made of a special quartz glass that aids the UV lamp in maintaining the correct heat and to protect the bulb from the water. This sleeve is mounted in the hole in the end cap with a compression ring. The inside of the canister is called the reaction chamber. There may or may not be a protective sleeve insert for the inside wall of the canister. This would be to protect the inside walls from becoming damaged due to the UV rays and becoming brittle. Again, there are many different manufacturers; therefore, you will have to follow their instructions for the type of unit you have.

Maintenance on a UV Sterilizer should be performed on a bi-weekly basis. Just as the inside of your return line gets lined with slime and becomes opaque, the inside of the UV will do the same. This will affect its ability to do its job. To clean a UV Sterilizer, you will have to be familiar with the inner workings of one. It is a simple design; however, there are a few parts that will need good cleaning. Make sure the unit is unplugged from the power source before attempting to do any maintenance. Most all UV Sterilizers for aquaria are designed with a hole or window that you can see the glow of the bulb while it is running.

When you remove the UV from the system, take it apart carefully and clean it inside and out with a mild bleach and water solution. After good and clean, rinse well and re-assemble. This would be a good time to inspect everything, and make note of anything that may need replaced due to deterioration. Many experts advise to replace the bulb after 6 months of use. Most manufacturers suggest at least replacing after 1 year, however, it has been noted that most UV lamps lose the ability to kill after about 9 months. Changing the bulb every 6 months will assure that it is going to work without flaw.

Ok, all of this talk about UV Sterilizers is ok, but what does it kill? Since organisms are exposed to the UV light, the ultra violet rays cause a change in the chemical bonds of the DNA (deoxyribonucleic acid), preventing their multiplication, which will result in annihilation of the specie. Only microorganisms, like algae, parasites or viruses, floating in the water and exposed to the UV light are eliminated. Nitrifying bacteria contributing to the well-being of fish are not carried to the sterilizer, and therefore do not come in contact with the UV light and will not be destroyed. There have been some findings that the Amphipod populations are threatened by the use of UV Sterilizers. Although some of the larvae from the Amphipods and other benthic creatures could be waterborne, they usually lay their eggs in the substrata and in the rocks crevices and a very small amount would be released in the water column to be killed by the UV Sterilizer.

Note: In the case of using a refugium, I recommend placing the UV Sterilizer on a different return line altogether. Refugiums are usually designed to be pod farms and food sources for the main tank. Placing the UV after the refugium, would possibly kill the pods before making it back to the main tank.

I would like to close by saying that although UV Sterilizers have caught somewhat of a bad rap these days with other methods of eradication of parasites becoming easily added and less harmful to the fish and invertebrates as they used to be, I have a UV Sterilizer on my tank, and it runs 24 hours a day, 7 days a week. I have not had an outbreak of any parasite that I am visually aware of. My amphipod and benthic population is thriving, and most of all, my fish are happy. This makes me happy. So, if you want a UV Sterilizer, and are willing to maintain it, by all means get one. You won’t be sorry you did.


Martin A. Moe Jr., The Marine Aquarium Reference “Systems and Invertebrates” Green Turtle Publications, 1989

Gregory Skomal, Setting Up a Saltwater Aquarium; Howell Book House Publications 1997

Vincent B. Hargraves, The Complete Book Of The Marine Aquarium; Salamander Books 2002

Filed under Articles, Saltwater.