Ultraviolet sterilization sounds like it’s a procedure doctors perform to cure only the worst medical conditions. Fortunately, it’s nowhere near that scary.

UV sterilization is in fact a very simple process for removing unwanted water borne bacteria, parasitic, fungal, viral, algae and other unfriendly pathogens out of aquarium water by exposing it to high intensity UV light.

The fear of a disease outbreak occurring in the aquarium is one many aquarists share, and understandably so. Oftentimes disease originates in a home aquarium with a single carrier whose condition worsens, eventually effecting the entire population. Other times there is a combination of reasons behind the disease, such as bringing home a sick fish from a local fish store, inadequate acclimation, not quarantining new species and poor aquarium husbandry, to name a few.

Let’s have a look at the “new fish” scenario.

An aquarium hobbyist is perusing his/her local fish store and finds a new specimen to add to their home aquarium, which presently houses a colorful assortment of species that have been collected over the years. Fortunately this particular species is compatible with his/her current livestock and there is plenty of space is available, so overcrowding isn’t a problem. Once home, the fish is carefully introduced and the aquarist is, for a time, very happy with the new inhabitant.

Soon the new fish begins to exhibit strange behavior, rubbing and flicking its body against live rock, almost as if it were attempting to satisfy an itch. Upon closer inspection, white spots are visible on the boy and fins of the fish. This could be what is known as White Spot; the flicking may be something else entirely, such as the dreaded Marine Velvet. Worse yet, the standard treatment for these two problems—a simple dosing of cooper—cannot be accomplished in a reef aquarium with live rock since the cooper can be deadly for corals.

Catching fish in a reef aquarium is always difficult, if not impossible. Commercial traps may be utilized, but time is of the essence.

The aquarist spends the next few days carefully monitoring the other fish in the aquarium, a few of which now have white spots of their own. It is clear the disease is spreading. Two of the fish are struggling to breathe, and the breaths they take are in rapid succession. The next day two fish are dead, and the others are still in jeopardy.

All this because one fish became infected!

The scenario described above can—and often does—happen. However, all is not lost.

The main reason fish become infected with disease at home or in the pet store is stress. All fish are—to an extent—under stress at various times on a daily basis, whether in the wild or in the home aquarium. The stressor may be fear of predation, lack of food, territorial struggles or not finding a suitable hiding place.

Arguably the most stressful time for a fish is when it is captured in the wild. It will be bagged and transported by land and air for many hours in total darkness only to be debagged and bagged once again once it reaches its destination. The journey from the reef to the wholesaler, the wholesaler to the retailer and from the retailer into your aquarium is undoubtedly a wild ride, to say the least.

During this transition and even after acclimation, fish are very susceptible to health issues. All of the stress the fish has been under depresses the immune system, allowing the aforementioned harmful organisms to run amok. If the new fish is infected, the fish in your aquarium are already in danger. On the wild reef a fish may, or may not, meet parasites like those described in this article. If one is met it is unlikely to cause severe problems – the reef is very large with many fish. In the confines of an aquarium, however it is an entirely different matter.

So what can be done?

Ultraviolet (UV) light can be used in the aquarium to ‘disinfect’ the seawater, preventing disease and helping to clean the water if disease is already present.

You may notice a bank of UV sterilizers in your local fish store. Since fish stores generally have several aquariums and fish are always coming and going, as a precautionary measure there must be a reasonably effective way of reducing the risk of disease.

In the home aquarium, there is only the occasional new fish but the UV sterilizer can still be a useful "anti-disease" weapon for the aquarist.

UV sterilizers are usually in the shape of a wide tube, the detail varying according to manufacturer. Inside the unit is an ultraviolet fluorescent lamp that emits light at 254nm.

The lamp is constructed of quartz glass, which helps stop the UV light intensity from being reduced as it would if normal glass were used. The lamp inside its own container to protect it from the water that passing by. This passing water moves through a narrow space so that maximum penetration of the UV light is achieved. The water flow going past the lamp is created by a powerhead or pump. The water needs to flow past the lamp at a desirable speed (generally specified by the manufacturer) in order for the UV light to work.

It is important that a correctly sized UV sterilizer be obtained; the capacity is related to the size in net gallons of the aquarium system. This capacity should be the whole system, including aquarium and sump. The manufacturer will have marked the recommended maximum capacity for the unit – if the aquarium system is a little over this, it is best to obtain the next size up. Some over-capacity is not a problem.

Aquarists may be concerned about the welfare of their bio-filtration – the life-supporting bacteria in all aquariums. UV sterilization unit will not harm these bacteria; the UV light is wholly contained within the unit.

The primary job of the UV sterilizer when fitted to an aquarium is the destruction of minute free swimming organisms that can parasitize fish, along with any other free swimming unwanted organisms that could potentially cause disease.

To understand how the UV sterilizer can kill organisms that appear to be attached to the fish, a further look at the organisms causing White Spot and Marine Velvet is needed. As discussed, parasites attach to the fish. After a period they fall off onto the rocks, etc. and encyst. When encysted they multiply and, when ready, free swimming parasites are released into the water. At this point, they are looking for a fish to act as a host. If they don’t find one, they will die.

In the confines of an aquarium, it is more than likely that the majority of the free swimming parasites will find a host. This is the problem—the parasite population can reach enormous numbers as they repeatedly encyst and release evermore parasites to infect the fish. In this situation, the fish will eventually die.

Not a very heartening picture, but there is good news.

The parasites have to go through a free swimming stage, and during this stage they are vulnerable. It is during this stage that the conventional copper treatment is effective, and the same applies to UV sterilization.

When the parasites pass by the UV light, they are either killed outright or are badly damaged and no longer a problem. It is important that the parasites receive the correct amount of radiation, and that is why the manufacturer’s instructions for pump size must be followed.

There is a potential downside to this: if free swimming parasites can be killed, then is anything good also killed?

Unfortunately, the answer is yes, but it is not necessarily a problem. If the aquarist is fortunate enough to have plankton, then if this passes through the sterilizer it will be killed. Plankton is not usually present in any quantity in an aquarium, so that is why it is not a problem. Any other minute beneficial organism will meet the same fate, although the bacteria employed for bio-filtration are not at risk as they are not free swimming but housed in media. If the fish are fed live food then it is best to turn off the sterilizer for the feeding period. If the sterilizer is being used because of a disease problem, then it is best to allow it to run.

Setting up a UV sterilizer is quite simple. The unit needs to be placed in a convenient and accessible spot, such as the cupboard in an aquarium stand. A suitable pump goes into the aquarium or sump, preferably the aquarium, where it is more likely that parasites will be drawn into it (parasites will be present in the sump but not as many). Its presence can be camouflaged but it is essential that the intake is directly open to the aquarium water, and not under any rocks etc. Standard flexible connection tubes are required, one from the pump to the unit and then from the unit to the aquarium. The pump is better placed at one end of the aquarium and the outlet at the other.

You might be wondering if you should run the sterilizer continuously or just every now and again. The benefit of running a sterilizer every so often is in doubt unless an electric timer is incorporated and the unit runs regularly during the daytime or nighttime hours. Using the unit only when it is remembered is just not good practice. Using the unit continuously is not harmful plus it removes the need for timers. Between these two options, running continuously would seem to be your best bet.

UV sterilizers can be of use on any type aquarium, fish only, coral reef or mixed reef. The treatment or medication of choice for the diseases mentioned earlier remains copper, and this should be the first weapon. The fact that copper is deadly to corals prohibits its use in a coral or mixed reef. This is where the sterilizer is of most benefit.

If the aquarist has the misfortune to be faced with a parasitical disease, a UV sterilizer can be of considerable use. It can even be combined with ‘reef safe’ medications, if desired. The aquarist must be sure, however, that the medications are safe for use in the presence of a sterilizer. In this instance, the manufacturer’s instructions should be followed or their advice sought.

A UV sterilizer is not very expensive, doesn’t cost much to run and there are few demands it makes upon the aquarist.

First, it must be correctly sized for the capacity of the aquarium system. Second, it must be outfitted with the correct sized pump. Third, the UV lamp must be changed at the proper intervals. If the lamp is not changed, the UV radiation will diminish and the whole point of having the unit will be negated as it will no longer be effective against parasites. A good idea is to put a label on the unit so you’ll remember to change out the lamp when the time comes.

The aquarist is responsible for ensuring, as much as possible, that the fish and other livestock in the aquarium are not diseased or at risk of disease. This means careful selection of livestock and careful introduction using a quarantine facility. Overstocking should be avoided and the habitat maintained at a high level. Feeding should be appropriate to the livestock and not overdone.

With the above in mind, a UV sterilizer is of good use as another layer in the aquarium defenses.