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Using UV Sterilizers in Backyard Ponds

Want less algae in your backyard pond? Use a UV sterilizer.

By David Lass |

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Ultraviolet (UV) sterilizers are important in outdoor ponds and marine (reef) tanks, and I consider them pretty much a necessity. Ultraviolet sterilization has been known for many years, and it is used in a wide variety of applications. Ultraviolet light is beyond what humans see as visible light. It is on the light spectrum (which I have always remembered using the mnemonic device of "Roy G. Biv”: red, orange, yellow, green, blue, indigo, violet) beyond the wavelength of violet. Ultraviolet light has industrial applications, such as for curing plastics or killing bacteria. Ultraviolet light is dangerous to human beings, as looking directly into it can cause blindness (that’s why they tell you never to look at a solar eclipse directly as the UV rays are still coming at you). Ultraviolet radiation is what causes sunburn when we have been exposed to the sun for too long.


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Ultraviolet sterilization for use in ponds works by passing the water past a UV lamp. The lamp is in a sealed glass or quartz tube. The entire UV apparatus is sealed in a closed vessel, and the light is only visible through a small port, so you can tell if the UV lamp is on. The actual water does not come in contact with the UV lamp, but the water and anything in it is exposed to the UV rays. For our aquatic purposes, those UV rays kill unwanted things in the water stream — primarily bacteria, protozoan spores and algae. Ultraviolet sterilizers are sized for different applications in varying wattages of UV lamps. The flow rate (usually called "dwell time”) of the water past the UV lamp is specified, and having the correctly sized UV sterilizer and correct dwell time is very important. Theoretically, UV sterilization should work like a leak in the bottom of a swimming pool (no matter how small the leak and how big the pool, all the water will eventually drain out) or a gambling casino (the house always has an edge, and eventually you will lose all of your money). Theoretically, as long as all the water is passed over a UV lamp, eventually everything in the water will be killed off. While this may be true, it is important to size the wattage and flow rate properly (more about that later on when I’ll discuss how to use UV sterilizers).


One of the best ways to ensure pristine water in a backyard pond is through the use of a UV-sterilizing unit. These units kill virtually all waterborne elements, such as free-floating algae, that pass through them.

There are two primary applications for UV sterilizers:

  1. Clearing green water. Ultraviolet will kill off all of the algae which cause "green water” that are in suspension in the water column. It is almost magical how UV quickly and completely does this.
  2. Killing nasty things in the water column. Bacteria, free-floating stages of parasites and protozoans and such are killed by UV. It is important to stress the fact that only the free-floating stages that are in the water column are killed. Ultraviolet is not a miracle cure for all diseases, parasites and protozoans.
In ponds, UV is really the only way to keep the water crystal clear and the fish visible all the time. Pondkeepers resort to all sorts of devices for dealing with green water, but eventually the only thing that may really work is using UV.

Use UV to Clear Green Water

Algae is part of keeping fish. I always preach that a little algae is good for a tank or a pond. Even algae growing on plants, rocks, etc., is to me a part of the natural flora. When it comes to algae in suspension in the water — free-floating green algae — we are talking about "green water,” and at that point the algae has become a major nuisance and must be addressed. Like any unwanted or nuisance algae, green water in ponds comes from too much light and too much food — but mostly from the light.


A UV sterilizer can have your pond looking its crystal clear best in no time at all.

UV and Fish Health

The other major use of UV sterilizers (both for ponds and marine tanks) is to control diseases, protozoans and parasites. In ponds, a number of protozoans and parasites have life cycles similar to algae; by killing off the free-swimming stage, the infestation is kept in control. It is important to remember that the UV can only work its magic on nasty things that are in the water column and that pass the UV lamp. While UV can dramatically improve the water quality and clarity, if a fish has a bruise, missing fin or fungus, UV will not do anything.

Ultraviolet is also often recommended to control bacterial infections in fish, and I am not at all convinced that this really works. Ultraviolet will most certainly wipe out all — or most — of the bacteria that are floating in the water column. The problem is that most bacterial infections are on the fish themselves, and while UV may improve things to some extent in the water column, it really cannot do anything for bacteria on the fish. But this is a good thing about UV: it can be used for ich or green water without doing any damage to the beneficial bacteria living on surfaces and that drive the nitrogen cycle.

UV and Aquariums

The standard suggested way for treating green water in a fish tank is to keep the tank in complete darkness and to not add food to it for at least three consecutive days. Sometimes this works. And cleaning the filter, cutting down on light, and reducing the amount and frequency of feeding may keep green water away. Usually, it will not, and unless the hobbyist really changes things, the green water often comes back.

Using a UV sterilizer on a green-water tank will fix the problem in a couple of days. When I had my pet stores, I always used to keep a UV sterilizer available for rental, specifically to handle green-water problems. Many times when hobbyists saw what the UV did for the clarity of the tank, they would choose to buy one. For others, we worked out a multiple rental deal where they could use the UV for a couple of days a month.

Ultraviolet sterilizers are excellent pieces of equipment for use in the aquarium hobby, but we have to be careful that we do not rely on them to solve husbandry problems while we neglect the basics of how to take care of fish and tanks. Ultraviolet is used frequently in marine tanks; in reef tanks, UV is almost a requirement, as sensitive reef invertebrates, such as corals, clams and shrimp, cannot tolerate most of the medications used to cure or prevent diseases in fish.

Another one of the primary uses for UV sterilization in an aquarium —freshwater or saltwater — is to keep the ich protozoan in check. Ichthyophthirius multifiliis (Cryptocaryon irritans in marine fish) shows up as little white spots on the fish. These protozoans have other stages where they swim freely in the water, and then encyst in the substrate and swim around until they infest a host fish. Ultraviolet sterilization is only effective on these protozoans when they are in the free-swimming stage. If the UV sterilizer is sized properly, it should kill all of the free-swimming stages and thus eventually completely wipe out the ich.

Ultraviolet sterilizers come in two different configurations. For small tanks (and here I would mean up to 50 gallons or so), there are UV sterilizers that are combined with a mechanical filter and that operate completely inside the tank. These have the advantage of not having to mess with any plumbing, and the pump is sized for the aquarium each unit is designed for. Inside-the-tank UV units work very well; however, they work best if there is another filter that takes primary care of mechanical filtration. Larger systems can utilize out-of-the-tank UV options that are unobtrusive, commonly sold and work well.

How to Use UV Sterilizers

The most common configuration of a UV sterilizer is one that sets up outside the tank or pond. A separate pump is required and sized for the proper flow rate for the pond it is being used on, and water is piped from the pond, through the UV sterilizer and back into the pond. It is imperative that the water be flowing past the UV lamp before you turn it on — otherwise, you can ruin the lamp. The dwell time of the water as it passes over the UV lamp is very important, and the manufacturer’s specifications for flow rate should always be followed. Also, if possible, it is a good idea to hook up some kind of simple mechanical filter in front of the pump that is pushing water through the UV. The cleaner the water flowing past the UV lamp, the better the results will be.

Regular maintenance is also important with a UV sterilizer. This consists of cleaning the quartz sleeve that encloses the UV lamp on a regular basis and also replacing the UV lamp with a new one on a regular basis. Again, follow the manufacturer’s specifications, usually given in terms of run hours, for how often you need to replace the lamp. Ultraviolet lamps must be changed at the specified intervals, as their effectiveness drops off rapidly beyond their design life. One good friend who has UV units in his store for his fish-selling tanks complained that he was having problems with his fish breaking out with ich. I asked him when he had last changed the UV lamps, and his eyes lit up with the shock of realization. New UV lamps solved the problem.

My Experience With UV Sterilizers

When it comes to outdoor ponds and green water, a UV sterilizer really helps to ensure a crystal clear pond. I have a 3,000-gallon outdoor pond. I used a pond filter that included a UV sterilizer, and I really could see the fish all the time and all the way down to the bottom of the pond, which is almost 6 feet deep. Many pond filters you will see today include a UV sterilizer already built into and integrated with the mechanical filter. The pump that runs the pond filter is sized for the proper dwell time for the filter, and the wattage of the UV lamp is proportional to the size of the entire filter and pump. I have used three different brands of pond filters and UV combinations (one of my regular gigs is to test and review products for my column "In The Fishroom” in Aquarium Fish International), and they all worked beautifully. Without using a UV sterilizer in an outdoor pond, you may always be fighting a losing battle against green water.

UV sterilizers are great equipment for use in pondkeeping. For ponds and reef aquariums, they are often the only thing that will control green water in a pond. There is a wide selection of products to choose from — but make sure that you replace the UV lamp at the specified interval.

David Lass has been an avid hobbyist for many years.
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