Aquarium UV lamp: how long should it be turned on each day and for how long as a treatment?

A UV-C sterilizer is often purchased in an emergency, when aquarium water turns pea green or becomes cloudy. Once it is connected, the same question almost always comes up: should it be left on day and night, for a few hours, or only until the water becomes clear again? In practice, an aquarium UV usually operates continuously, 24 hours a day, during a treatment lasting from a few days to two weeks, or permanently when used for prevention; the lamp itself is generally replaced after about a year of continuous use. Here are the guidelines for adjusting the duration according to your objective.

How a UV-C sterilizer works

A UV-C sterilizer passes water through a chamber where a lamp emits short-wave ultraviolet radiation, around 254 nm. This radiation damages the genetic material of suspended organisms: single-celled algae responsible for green water, free-floating bacteria responsible for cloudy water, and some parasites and pathogenic germs at the stage when they swim freely. The “dead” algae then clump together and are trapped by the filter.

Two important consequences follow. First, UV only acts on what passes through the unit: algae attached to the glass, plants, or décor are not affected. Second, it does not treat the cause: if the water turns green, there are too many nutrients and too much light. UV makes the symptom disappear, but the cause must be addressed at the same time. Our articles on aquariums that turn green and cloudy aquarium water explain these causes in detail.

How many hours per day?

The answer depends on how you use it. Three situations occur most often:

In the vast majority of cases, an aquarium UV unit is designed for continuous operation. Turning it off at night offers no particular biological benefit: suspended algae also multiply at night, and every pass of the water through the unit counts.

UV-C lamps are fluorescent tubes: repeated switching on and off is what mainly wears out their electrodes. Programming a UV unit to run for ten cycles a day to “save” the lamp often has the opposite effect.

How many days does it take to clear green water?

With a properly sized unit and an appropriate flow rate, green water generally clears within a few days. The first improvements are often visible as early as the second or third day; water that is truly “pea soup” may take one to two weeks. If nothing changes after a week, there is no point waiting longer: something is wrong.

The most frequent causes of a treatment that does not progress are:

• Flow rate too high in the sterilizer: the water is not exposed for long enough. The manufacturer specifies a maximum flow rate or a flow-rate range; beyond that, effectiveness drops.
• Insufficient power for the aquarium's volume.
• A fouled quartz sleeve that filters the radiation.
• A lamp nearing the end of its service life: it still lights up, but emits almost no UV-C.
• A persistent cause: excess food, lighting left on too long, aquarium in direct sunlight.

Flow rate: the real setting that determines effectiveness

People often focus on wattage, but contact time determines effectiveness. The more slowly the water passes in front of the lamp, the greater the UV dose it receives. Conversely, a very powerful external filter connected to a small UV unit may move the water too quickly for the treatment to be effective.

In aquariums, a common approach is to pass the water through the UV unit at a moderate flow rate, roughly one to two times the aquarium's volume per hour for action against algae, and even more slowly when targeting bacteria and certain parasites. These figures are guidelines: the device instructions take precedence. If your filter's flow rate is much higher, a bypass with a valve allows you to send only part of the flow through the sterilizer.

When to switch off the UV

There are several times when the sterilizer should be switched off temporarily:

• While adding bacteria when starting an aquarium or after cleaning: the live bacteria introduced into the water would pass through the UV unit. It is generally switched off for 24 to 48 hours, depending on the instructions for the bacterial product.
• During certain medication treatments: UV light can degrade some active ingredients. Follow the medication instructions.
• With certain plant fertilizers: chelated iron, for example, can be degraded by UV light. In a heavily planted aquarium, many aquarists schedule the fertilizer and UV so they do not overlap, or reserve UV use for treatments.
• During any maintenance: unplug the equipment before opening it or touching the hoses.

On the other hand, UV does not interfere with the nitrogen cycle in an established aquarium: beneficial bacteria live attached to the filter and substrate, not suspended in the water.

After how long should the lamp be replaced?

This is the trick question. A UV-C lamp continues to glow long after losing most of its effectiveness: the visible bluish glow is not germicidal radiation. Most manufacturers therefore recommend replacing it after approximately one year of continuous use, or around 8,000 to 9,000 hours, even if it appears to be working. For a UV unit used only for occasional treatments, count cumulative operating hours instead.

Note the installation date on a piece of adhesive tape stuck to the device: it is the simplest way not to forget. To check whether a device is still actually working, our dedicated article on sterilizer checks, presented at the end of the page, details the necessary checks.

Quartz sleeve maintenance

The lamp is protected from water by a transparent quartz sleeve. Over time, limescale or biofilm builds up on it and blocks some of the UV radiation. Checking it every three to six months, depending on water hardness, is a good routine. Clean it with a soft cloth, optionally moistened with a little white vinegar to remove limescale, then rinse it thoroughly. Take the opportunity to check the condition of the O-rings: a worn O-ring is the most common cause of leaks under the cabinet.

Handle the new lamp with gloves or a clean cloth: fingerprints on the glass reduce its lifespan, as several manufacturers point out.

The duration also depends on the selected power

An undersized sterilizer can run for weeks without clearing green water, whereas a properly sized model can solve the problem in a few days. Manufacturers specify a maximum aquarium volume for each device, often varying depending on whether the goal is to target algae or microorganisms. To act against bacteria and parasites, the required UV dose is significantly higher than for algae: in that case, choose a more powerful device or run the water through it more slowly.

In practice, it is better to use a slightly oversized device with a moderate flow rate than a small model through which water flows too quickly. If you are hesitating between two power ratings, choose the higher one: the treatment period will be shorter, and the device will retain some capacity when the lamp ages toward the end of the year.

Safety: rules you should never forget

• Never look at an illuminated UV-C lamp, even briefly: the radiation is dangerous to the eyes and skin. Quality devices have a filtered inspection window to check that they are working.
• Never operate the UV unit out of water or without water circulation: the lamp heats up and the device may be damaged.
• Always unplug before opening, cleaning, or changing the lamp.
• Create a drip loop in the cable and protect the plugs from splashes, as with any aquarium electrical device.
• Take used lamps to a collection point: they are fluorescent lamps that must be recycled.

Permanent UV treatment or treatment in courses: which should you choose?

Both approaches are valid. Permanent UV treatment provides remarkably clear water and reduces the load of free-floating microorganisms, which is particularly useful for heavily stocked tanks, quarantine tanks, and saltwater aquariums. UV treatment in courses is suitable for balanced, well-planted aquariums that experience only occasional episodes of green or cloudy water, for example after cycling or changing the lighting.

In any case, an aquarium whose water stays clear only thanks to UV treatment deserves a closer look at its inhabitants, lighting, and feeding. The long-term goal remains a stable tank, as explained in our guide to achieving crystal-clear water in an aquarium.

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Frequently asked questions

— Should you leave the aquarium UV lamp on at night?

Yes in most cases: the UV sterilizer is designed to operate continuously, and suspended algae also multiply at night. Repeatedly switching it on and off wears out the lamp more quickly.

— How long does it take to clear green water with a UV sterilizer?

Generally a few days with a properly sized unit and an appropriate flow rate, sometimes one to two weeks for very green water. If there is no improvement after a week, check the flow rate, power, sleeve, and lamp.

— When should you change an aquarium UV lamp?

Most manufacturers recommend replacing it approximately once a year with continuous use, even if the lamp still lights up, because its UV-C radiation decreases over time.

— Should you turn off the UV when adding bacteria to the aquarium?

Yes, generally for 24 to 48 hours, depending on the bacterial product's instructions, so the bacteria have time to settle in the filter.

In summary

• An aquarium UV sterilizer generally operates continuously, whether for treatment or prevention.
• Green water clears within a few days, or one to two weeks at most with a properly adjusted unit.
• Flow rate matters just as much as power: if it is too high, the water is not exposed long enough.
• Turn off the UV when adding bacteria, during certain treatments, and for any maintenance.
• The lamp should be replaced approximately once a year with continuous use, even if it still lights up.

Last updated: October 3, 2026.