14.1. UV Sterilizers

UV sterilizers are small electrical  light treatment units which have water flowing over a special electric light bulb. This special bulb puts out an invisible high energy type of light called UV light. UV light is what causes sunburns in humans exposed to the sun for too long. This UV light kills micro-organisms in the water passing over the bulb.

UV light should never be directly viewed by humans. The UV light can result in severe damage to ones eyes even with short duration exposure.

A typical UV unit looks like this:

Diagram of the innards of an Aquarium UV Unit
Aquarium UV Unit

The aquarium water is pumped, typically by a canister pump but sometimes by a separate powerhead, through this unit from one end to the other end. As the water flows by the light bulb the living organisms in the flow of water are killed by the UV light. The UV light is always accompanied by some violet visible light which can be seen sometimes though view holes.

Note that UV ONLY kills organisms that pass over the bulb. It does not kill any beneficial bacteria as beneficial bacteria only grow attached to surfaces, not free swimming. And UV does nothing to brown or green algae growing in the aquarium on decoration, gravel or rocks.

Aulonocara koningsi Blue Regal Mbenji
Aulonocara koningsi Blue Regal Mbenji

Benefits of UV Sterilizers

UV does the following:

  • Kills algae, infusoria and some disease organisms like ich, in the water column (and only in the water column, not on surfaces in the aquarium). If the turn-over is more that four times per hour through the UV it will also control bacteria.
  • The aquarium filter will then remove the dead bodies, sometimes in hours, sometimes in days.
  • Because some of the organisms in the water column are dead and no longer multiplying, the number of some organisms in the water column decreases.
  • Because the immune systems of the fish no longer must fight off some of the organisms in the water column, the fish have more immune system resources available to fight off pathogens.

Note that a hobbyists should not count on UV for reducing the count of some types of bacteria. Some bacteria reproduce very slowly, on the order of splitting in two every two days. These types of bacteria can be controlled with UV. But there are some bacteria that can replicate every ten minutes in polluted waters. Trying to control these bacteria via UV can be difficult if not impossible. So many bacteria cannot be controlled by by reasonable amounts of UV and reasonable flow rates. They have to be controlled by biofiltration.

Also note that UV is NOT a substitute for good biofiltration. The amount of surface area available in the filter is many times more important than a UV unit and will do a much better job of keeping fish healthy.

Fossorochromis rostratus
Fossorochromis rostratus

UV can be looked at as a type of filtration and filtration should be looked at from a chart.

Filtration Options
Filtration Options

Obviously biofiltration is easily 90% of the game here. But ultraviolet sterilization, while technically not a form of filtration, does a very good job of “filtration”.

Note that UV treatment cannot eliminate dissolved organic compounds (DOCs). Only biofiltration can eliminate DOCs. Since DOCs are what feed bacteria in the water column it is important to eliminate them to produce really crystal clear water that is free of bacteria. So UV needs to be accompanied by very good filtration with good media like foam or pot scrubbers to produce crystal clear, bacteria free water.

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UVC Light Spectrum
The “UVC” in the above diagram is the invisible type of light which sterilizes organisms.
Aulonocara stuartgranti Maisoni Chitimba
Aulonocara stuartgranti Maisoni Chitimba

Operation of UV Units

UV units should be left on 24 hours a day, seven days a week. One needs to maximize the amount of UV light and keep the algae and disease organisms at bay ALL the time.

Also the UV lights are a type of fluorescent bulb generally. Fluorescent bulbs often fail when turned on and off frequently. The instructions that come with most units say to leave them on for only 3 to 4 hours a day. The manufacturers of the units know that this will burn out bulbs quickly. So they make more money replacing bulbs. Don’t you just love the profit motive?

Aulonocara rostratum
Aulonocara rostratum

A Facebook comment is pertinent. From one John Dobs:

“At one point, my UV was on the same timer as the lighting system. I was replacing the bulb about every 6, maybe 8 weeks. I have since taken it off the timer and manually turn the unit off anytime I do a water change of more than 50% as it is low in the tank… this is rare. If I were to guess… the unit has run for 4 months or better with the same bulb.”

It is important to clean the UV units on about a monthly schedule. Any slime build-up in the unit will interfere with the UV sterilization. The in-line units need to be after a filter, not before it. This prevents fish feces and debris from blocking the UV rays.

With 24/7 operation replace the bulbs once a year. The bulbs typically only take a year till they are producing half the UV output they had when new.

If the water has a high GH, i.e. high calcium levels, then the quartz sleeve can get a white deposit on it. This white deposit needs to be very carefully cleaned with concentrated hydrochloric acid (muriatic acid or pool acid) as the deposit can severely reduce the efficiency of the unit.

Note that UV should not be used to compensate for poor filtration. UV should only be used as an adjunct to good filtration. One common reason for “needing” a UV sterilizer is cloudy water caused by using a hang-on-back or in tank cartridge filter.

Aulonocara saulosi Green Face Peacock
Aulonocara saulosi Green Face Peacock

A hang-on-back filter or an in tank filter, especially if it is a cartridge type where the cartridge is cleaned or replaced weekly, will be a very poor biofilter. Such a cartridge filter needs an under-gravel, canister or sump added to it to give it good biofiltration.

UV Sterilizers for the Aquarium
UV Sterilizers for the Aquarium

Selecting a UV Unit Size and Flow

Wide spread social media consensus (including profit driven internet directions and the directions that come with the units), is that UV in the home aquarium needs to have high wattages, low flows, high dwell times, splitters and/or flow control valves to be very effective. This is simply incorrect.

Low wattages and low residence times are both very effective because of an obscure scientific principle called the “Reciprocity Rule in Photobiology” (a residence time of one second in ten passes has the same effect as a residence time of ten seconds in a single pass). It is based on the fact that aquarium water will pass though a UV unit many times, so any pathogen is exposed to the UV light many times. Roughly one watt per ten gallons of aquarium water is enough to kill even ich trophonts, even at high flow rates and low residence times.

When we say “obscure” we do definitely mean “OBSCURE“. Ninety nine percent of the social media commentators on using UV in ponds or aquariums know nothing about the “Reciprocity Rule in Photobiology”. You will find many papers on UV in the aquarium that obviously don’t even know about the rule.

Aulonocara sp Bicolor Peacock
Aulonocara sp Bicolor Peacock

All research on UV in water systems is based on single pass sterilization, where high wattages and long residence times are very important. In an aquarium there are multiple passes of a given volume of water through a UV unit, not a single pass. So aquariums have to be treated much differently.

How to set up an aquarium UV unit
How to set up an aquarium UV unit (photo from American Aquarium Products)

There is an excellent human analogy.  Sunburn is caused by exposure of the skin to UV rays. The experts will correctly tell one that you can not get a sunburn from being under a sun lamp for ten minutes, i.e. “the residence time is too short”.  But the human analogy of an aquarium being turned over six times per hour is being under a sunlamp for ten minutes once an hour for 24 hours a day, seven days a week. That sort of regimen will give a human severe sunburn every time. By the same token that sort of regimen in the aquarium kills most waterborne organisms.

If one is interested in an in depth analysis with references on how the wattage requirement of one watt per ten gallons is arrived at read the following link:

14.1.1. UV in More Depth

Price is a debatable issue. There are those that always go with the “better” more expensive units. The surprising thing is that many of the expensive units ($400) use the same UV bulbs as the cheap units ($60). Since the performance of a UV unit is largely dependent on the bulb this is somewhat confusing.

Aulonocara sp Flavescent
Aulonocara sp Flavescent

There are many types of UV units available. This link will guide one through the choices:

14.1.2. Selecting a UV Unit


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Translated in dutch by : Joost Abrahams
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