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UV Sterilizers: How They Work

Drs. Foster & Smith Educational Staff
UV Sterilizer Selection Guide 
Compare UV Sterilizers 
FAQs: Reverse Osmosis Water 

Q. Are UV sterilizers a good replacement for biological and mechanical filtration?

A. NO. UV sterilizers are not a replacement for good biological and mechanical filtration, regular water changes, or proper control of the nitrogen cycle.

  Maximum Flow Rate to Control:  
UV Bulb Bacteria
& Algae
Parasites Max Gallons
8W 120 gph N/A < 75 gal
15W 230 gph 75 gph 75 gal
18W 300 gph 100 gph 100 gal
25W 475 gph 150 gph 150 gal
30W 525 gph 175 gph 175 gal
40W 940 gph 300 gph 300 gal
65W 1700 gph 570 gph 570 gal
80W 1885 goh 625 gph 625 gal
120W 3200 gph 900 gph 900 gal
130W 3400 gph 1140 gph 1140 gal
Though manufacturers' recommendations will vary, this chart provides a general idea of the wattage you'll need - and the proper flow rates to adjust your pump to – when using a sterilizer for controlling bacteria/algae and for controlling parasites.

A properly selected UV sterilizer can help clarify water, rid free-floating algae, and control outbreaks of harmful bacteria and certain parasites.

UV sterilizers work on the principle that special fluorescent UV lamps can effectively irradiate (kill) microorganisms in aquarium water when they are exposed to this light. UV light, at a peak wavelength of approximately 254 nanometers, alters the genetic material in the organism's nucleus and shortens its normal life cycle. Theoretically, UV sterilizers can kill viruses, bacteria, algae, and protozoa with no residual effects. However, the organisms must be free-floating and travel through the UV sterilizer unit for the sterilizer to work. It has no effect on organisms attached to fish or rocks because they do not flow through the unit and cannot be exposed to the UV light.

Before selecting a UV sterilizer, consider your primary objective. Do you want to help control free-floating algae or do you want to control parasites? By deciding this, you will be able to select the proper unit to achieve your intended results. Though most manufacturers provide a recommended maximum aquarium size for each model they make, the application and the efficiency of a unit are also determined by flow rate, as well as bulb age and wattage.

Flow rate
Shortening or lengthening UV exposure time (dwell time) determines the effectiveness and use of a UV sterilizer. Adjusting the flow rate through the sterilizer controls UV exposure time. In general, larger organisms will need a longer dose of radiation than smaller organisms. For example, a UV sterilizer set at a higher flow rate is capable of controlling bacteria and free-floating algae but may not be effective against parasites. Reduce the flow rate through the UV sterilizer to lengthen the time parasites are exposed to the UV light. With a slower flow rate, the same unit can now be used to help control parasites.

Bulb age
Lamp effectiveness declines with time. Compared to when it was new, your UV sterilizer will no longer produce the same results after months of use. Therefore, you may have to increase the dwell time (by lowering flow rate) to produce desired results. Replace the UV bulbs every 6 months, or per manufacturer’s recommendation, in order to maintain UV efficiency. Also, clean the quartz sleeve of the lamp regularly to remove organic buildup. A clean quartz sleeve allows better penetration of UV light and maximizes the efficiency of the unit.


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