POWERful “medicine”

Ultraviolet Treatment for Aquariums and Ponds

Just as in human medicine, aquarium treatments for pathogens and pests often have undesired side effects or may even put the “patients” at risk. Antibiotics may inhibit the beneficial bacteria, so that ammonia or nitrite levels rise. Medications for “ich” and “velvet” often stain aquarium sealant, damage live plants, and stress the fish. Some stronger treatments for larger parasites appear to walk a fine line between eradicating the parasites and eradicating the fish. In “balanced” aquariums such as reef or heavily planted tanks, even the mildest treatments may tilt the scales enough to cause problems.

While chemical treatments are sometimes necessary, there is a technique for controlling many unwanted microorganisms with minimal disruption: Ultraviolet Sterilization.

Ultraviolet light incapacitates microorganisms by deactivating their DNA, making multiplication impossible or killing the organism outright. It does this without significant change to the water chemistry, and directly affects only organisms that are exposed to the UV (specifically UV-C) rays themselves, so it is a very safe and clean process, especially compared to medication. UV can be effective in controlling a wide variety of microorganisms, and has been used as a disinfecting process in other fields such as medicine, as well as tap water and wastewater treatment for decades.

The two main uses for Ultraviolet light in ornamental fish culture are as “clarifiers” to reduce green water algae in ponds, and as “sterilizers” to reduce waterborne pathogens in aquariums. The same UV units can be used for either application; it’s only a matter of “zap dosage”. Each organism has its own tolerance to UV, based largely on its size and structure. The zap dosage, expressed in microwatt-seconds per square centimeter, for many bacteria is under15 thousand for waterborne microalgae about 20-30 thousand while protozoan parasites may require 45-90 thousand or much more. Thus, a UV setup that is suitable for zapping bacteria would need to be about twice as powerful to eradicate algae, and 3 to 6 times as strong to control smaller protozoan parasites. It may not be practical to attempt to control larger parasites with ultraviolet radiation, and parasites that multiply without spending significant time in the water column will obviously be unaffected.

Dwell time. As can be seen in the zap rate expression, both the power of the UV and the number of seconds of exposure contribute equally to the destruction process. In other words, if double the zap dosage is needed, it can be achieved by either doubling the amount of UV that hits the organism or making it stick around (or “dwell”) twice as long.

Dwell time itself is a function of the fluid capacity of the unit versus the flow rate provided by the pump. The slower the pump, the longer the dwell time, and the more effective the UV will become. Most aquarium and pond UV manufacturers provide suggested maximum flow rates for each model. However, there is broad disagreement among manufacturers on precisely how those rates should be calculated, and as such the maximum or optimum suggested flow rate may not be a very useful factor for comparison.

A much more equitable starting point for comparing models would be lamp output. UV units are rated in terms of the amount of power they consume, but what really does the job is output power. In typical low-pressure UV lamps, the output wattage is in the neighborhood of 1/3 of the rating – so a 25-watt UV lamp may produce only about 8 watts of UV power. The percentage may vary by brand and model, and some manufacturers appear to be much more willing or able to share this information than others.

Other factors also play a role in properly establishing optimum flow rates, although they are not easy to quantify, and are hopefully addressed by the designing engineers. These include:

Housing diameter. The bigger the diameter of the housing, the more water the unit will hold, and the longer the dwell time. On the other hand, the bigger the diameter, the farther some water will be from the lamp, and the less UV it will receive.

Lamp length. In a similar trade-off, longer lamp length means more travel or dwell time if all else is equal, but a shorter bulb with the same output (for example a “compact fluorescent” style bulb) provides more power on the shorter trip.

Quartz Sleeves. Many manufacturers encase the lamp in a special quartz glass sleeve for ease of maintenance and added safety compared to units in which the water flows directly over the lamp itself. Even the best quartz sleeves absorb a fair amount of UV radiation, so less reaches the water.

Dead spots. If the sterilizer’s design allows for areas of poor circulation, water in those areas will have much longer dwell time. However, that means that water that is flowing properly through the chamber must do so more quickly, and the dwell time there is reduced. For more consistent flow, some manufacturers make it a point to put inlet and outlet ports near the very ends of the unit, eliminating potential dead spots between the ports and the unit ends. At least one manufacturer addresses the issue by forcing the water through a tubular sleeve that spirals around the lamp.

Water clarity. Since many types of particles (including the microorganisms themselves) absorb UV, efficiency drops considerably if the water is turbid. This is especially an issue in ponds, where microscope algae can make the water resemble pea soup. Some manufacturers factor UV loss due to turbidity into their calculations.

End Of Life measurements. Brand new bulbs emit significantly more UV than those nearing the end of their careers. For best comparison, any statistics presented should be taken at EOL.

SIDEBAR: Other considerations…

A few other features don’t relate as much to performance, but are also worth considering when selecting a model:

Lamp life and cost. Some lamps lose output and should be replaced every six months, while others may last a year or longer. Since replacement lamps aren’t inexpensive, it’s worth considering replacement costs when selecting a model.

Ease of cleaning. The lamp and/or sleeve of a UV should be kept clean of any accumulations that will reduce UV transmission. Some units make this a simple task, while others seem nearly impossible to service.

Ease of parts replacement. The lamp must be replaced on a regular basis, and eventually a starter, ballast or sleeve might need to be exchanged as well. Some manufacturers have readily available parts that can be readily installed by the end user.

Positioning of ballast. Just as in aquarium light fixtures, ballasts are particularly vulnerable to water damage, and should not be positioned where it’s likely water will eventually get spilled.

Weatherproofing/submersibility. For pond applications especially, the unit should be well protected from rain and accidental immersion.

Sizing an Ultraviolet Sterilizer. This is again a matter of considerable debate among manufacturers, with some taking a more conservative approach than others. Already hampered by the broad range of views regarding flow rates, the matter is further complicated by many unknowns specific to closed systems. Much of the most reliable research regarding UV disinfection deals with open systems such as tap water treatment. In those applications, the water goes through the system but once, all organisms must be eradicated the first time, and when the water is purified, it moves out of the system.

In closed systems like aquariums and ponds, treated water is immediately mixed with untreated water, and the resulting mix is treated again and again, slowly becoming more “pure”. If you filled a 100 gallon tank at 100 gallons an hour through a sterilizer, it would take an hour to be completely full of sterilized water. But if you hooked that same 100 gph pump and sterilizer up to a 100 gallon tank, it would take about 9 hours and still only become 99.99% sterilized. And that’s assuming that whatever you’re trying to eliminate isn’t multiplying during that time. Then, the question arises if one really needs a 99.99% treatment rate, or would 99.9% or even 99% be sufficient.

All the controversy over flow rates and sizing leaves the pet store with quite a dilemma: whether to recommend conservative sizing and models or live on the edge and push the units with greater claims. Whichever route is chosen, there will likely be competitors in the area – or certainly competitors on the Information Superhighway – that will promote the opposite, so employees had best be prepared to make their case.

In ponds, where the targeted organisms are highly visible, one could begin with a manufacturer’s recommendation on sizing and flow rate, and if the water doesn’t clear, flow rate can be reduced or additional UV units can be added. In aquariums, however, the target organisms are not visible to the naked eye, and it may be prudent to just start with a very conservative approach. Selecting a pump with an hourly flow rate of about the same as the tank capacity (e.g. 100gph for our 100 gallon tank) would be a good rule of thumb. Then selecting a UV unit (probably 15 to 25 watts) that will conservatively handle that flow rate for parasite treatment should yield reliable results.

To improve clarity and boost UV performance, the water should ideally pass through some sort of filtration before reaching the sterilizer or clarifier. The filter media will need to be cleaned regularly so as not to reduce flow.

In situations where a single pump supplies the UV, feeds filtration and provides circulation, it may not be wise to reduce flow rate to increase dwell time. A separate smaller pump and filter can be added to operate just the UV, or the large pump output can be divided, and only a regulated portion of the water can be fed through the sterilizer.

Though UV sterilization has few negatives, there are some situations where it may be contra-indicated. Since it kills indiscriminately, UV may reduce desired plankton populations in natural-style reef aquariums, and could conceivably extend the break-in time of a new aquarium by destroying some of the beneficial bacteria (most stay firmly attached to gravel and other surfaces). In addition, there is some discussion that fish living in an aquarium with too few pathogens may have reduced immunity should pathogens eventually return. As hopefully everyone is already aware, UV light can also cause great retinal damage to human eyes, so maintenance must be performed with caution.

SIDEBAR: In-Store Applications.

Ultraviolet sterilizers and clarifiers can be installed on any individual store tank or pond, but they are especially appropriate when multiple tanks or pools share a filter system. A properly sized and installed UV system can dramatically reduce the spread of algae, bacteria and even protozoan parasites. In this application, the UV really needs a high kill rate on a single pass, much like the open wastewater systems that have been heavily studied, so the most conservative flow rates and sizing guides should be used. A single, very powerful UV can be used before the manifold that distributes the water back to each tank, or each tank can have its own smaller UV installed just before the return hardware.

This article originally appeared in

Pet Age Magazine