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         Ultraviolet Light In The Reef Tank



Whether or not ultraviolet light, in the form of so called sterilizer units, are advantageous or detrimental to reef aquariums has always been a point of contention. Some aquarists maintain an opinion that UV sterilizers are a cure all for almost anything that might ail a tank, others contend that UV light is dangerous and kills all beneficial organisms in the water column. I think the truth lies somewhere between the two extremes and depends on exactly what it is that you expect a UV unit to do for you and your reef system.

What is UV sterilization?

Ultraviolet light (primarily the UV-C part of the spectrum with a wavelength between 100nm and 290nm) effects the proper functioning of living cells. The actual structure of the cell's nuclear material, the DNA that controls how the cell reproduces itself, is damaged. This damage is so severe that the organisms are completely incapable of reproduction. As a result, a die off occurs. Under ideal circumstances (from our point of view), this continues until there are no living organisms (algae cells, for instance) remaining.

The effect of a sufficient amount of UV-C light, for an appropriate amount of time,  concentrated (for example) on a microbe infested area, will result in a sterilized surface. This process can be brought to play in an aquarium.

UV sterilizer design

Although UV sterilizers are available in a number of design types, only one, the dry-bulb design is applicable for use in aquariums. Here a UV lamp is enclosed in a transparent and water tight quartz sleeve. See the illustration from Emperor Aquatics, below:


                                                                                                                          © Emperor Aquatics, Inc.

You can see that the quartz sleeve is mounted, axially, inside a (PVC) tube. The PVC tube has fittings to allow water to be pumped into one end and then through the entire length of the tube where it then exits. The UV bulb is inside the quartz sleeve and sealed so that no water can reach the bulb (dry bulb design). It is important that the sleeve is only minimally larger than the circumference of the bulb so that the distance the UV light needs to travel to reach the water flowing past the sleeve is as short as possible. This provides a maximum sterilizing effect.

Installing a sterilizer for maximum effectiveness

It often happens that you'll come across a thread on one of the reefing message boards where the original poster complains about the total ineffectiveness of the UV sterilizer that he/she just bought. They write that they can see absolutely no effect on their tank(s). They then go one to say that the unit they purchased has a rating of 25 watts, or maybe only 8 watts. They advise that they have installed the unit on their 120gal (or even 220gal) system.

I have always considered such UV units to be no better than toys when employed with tanks of over 55 gallon capacity. They just don't have the output in UV-C required to do the job being expected of them - and are, therefore, a total waste of money!

A couple of years ago I came across a company that seemed to be willing to tell it like it is with regards to what can and can't be done with UV sterilizers. Emperor Aquatics, Inc. has a wealth of information pertaining to sterilizers on their website.

Reading through their literature, it becomes evident that a UV unit must be chosen with consideration of the size of the system where it will be installed and the purpose for which it will be employed. I use UV for three main reasons:

  1. I want to prevent disease organisms from circulating freely through my system.

  2. I want to be able to control bacterial blooms that may arise from dosing carbon of one type or another

  3. I have found the UV actually tends to raise ORP.

The effectiveness of UV light at ridding a water column of a particular type of unwanted organism is a function of:

  •  the strength of the bulb being used. This is normally measured in watts.

  • the distance the light has to travel to reach the organisms to be controlled. This is a function of the design of the sterilizer. The quartz sleeve that houses the UV lamp should be dimensioned so that these is very little space between the lamp and the sleeve.

  • the length of time the water to be sterilized is subjected to the  UV light. This, again, is a function of the design of the sterilizer, as well as the speed at which the water is pumped through the unit past the UV light.

  • the amount of water that needs to be treated. The less water needing sterilization, the more often the water can pass through the UV unit in a given period of time. The more often the water passes through the unit the more effective it will be at it's job.

The chart, below, can be used to determine which UV unit should be chosen for a particular purpose and environment. The chart includes data pertaining to outdoor ponds, but we can ignore this as the parameters are completely different for open bodies of water. Note that the data in the chart pertains to UV bulbs which are operating at 50% of their rated output (a three or four month old bulb).

I have highlighted (red line) the row of data which pertains to the model I use on my system - the SMART 80 Watt HO, model no. 025080-W/80. The unit uses an 80W high output bulb. The maximum water volume of the system to be treated should not exceed 600gals. The most interesting data here is the suggested maximum flow range. Notice that in order to sufficiently damage algae and bacteria, the flow rate can be 6 times the rate required for protozoans. For algae and bacteria the UV exposure should be approx. 30,000 micro watts per second per square centimeter, whereas for protozoans (Cryptocaryon irritans - saltwater ich, for example) the exposure should be approx. 180,000 micro watts per second per square centimeter!


                                                                                                                                                                                                                                                         © Emperor Aquatics, Inc.

In order to provide the kind of exposure illustrated here, the UV unit must be adequately dimensioned - both in size and output. Such devices are necessarily expensive.

Positive environment

To get the most out of the investment in a UV unit, certain conditions should be met:

  • don't use the unit when drugs or medicines are being dosed. This precludes their use in quarantine systems.

  • the water to be treated should be as clean (clear) as possible to ensure maximum UV light penetration.

  • the UV bulb should be replaced every 6 to 8 months.

  • the unit, especially the quartz sleeve, must be kept clean, for obvious reasons.



John says...
Great site! I have the same UV. Just installed it. I have a red dragon as a feed pump. So I have around 3K GPH. So far the effects on the tank are very positive, clearer water and less algae issues. What is your flow rate and what are you trying to accomplish? Do you vary the flow rate to achieve differnet goals? TIA.
GlassReef: I normally run my UV at approx. 1000 gph - I normally don't change the flow from this setting as I find that the tank tends to stay crystal clear (the water become almost "polished) and algae blooms never become a problem. That said, I did decrease the flow to about 350 during a bad bout with dinoflagellates.
9th May 2012 10:12am

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