How Long Does It Take A UV Sterilizer To Clear Green Water? {Does It Kill?}

Do you wish to explore UV sterilizers in clearing your green water? How long does it take a UV sterilizer to clear green water?

Are you hoping the UV sterilizer will clear your water as soon as possible?

This guide contains all essential details and information about using UV sterilizer to clear green water.

How Long Does It Take A UV Sterilizer To Clear Green Water?

A UV sterilizer only takes 24 to 48 hours to clear green water. It is possible to have free-floating dead algae after 24 hours. All you need is to:

  • give the UV more time
  • carry out about 50% water change

The use of UV light and UV sterilizer in the aquarium setting is immense. The green water in the pond is a result of too many algae in the pond. The UV sterilizer penetrates algae and destroys the cells.

Personally I have used a fluval UV sterilizer (Available on Amazon) in my aquariums and I have been very impressed how good they perform.

UV Sterilizer To Clear Green Water

Why Is My Pond Water Green?

Green pond often raises a lot of questions among pond owners and aquarists. The principal cause of a green pond is the presence of green algae.

  • Algae are a single-celled organism that can pollute pond water and cause it to become green.

The high amount of algae in the pond indicates that something is wrong with the pond quality. Algae often boom in ponds due to an imbalance in the level of phosphorus and nitrates. This imbalance supplies these organisms with all they need to grow.

Is A Green Pond Safe For Fish?

No. The imbalance of nitrate and phosphorus in the pond indicates low pond quality. This condition can be a result of overfeeding and overcrowding of the pond. It can also result when organic wastes accumulate in the pond for other reasons.

A green pond is not ideal or hygienic for your fish. The green color of ponds indicates poor quality, and it is best to use quality treatment methods.

The method should include evacuating the algae waste from the pond. If the remains of the algae fall back to the pond, it will cause more harm. It will still cause the water to be turbid and prevent clear water.

Does A UV Light Clear Green Water?

Yes. The use of UV light among aquarists is frequent because of its immense benefits. UV light is helpful for various purposes, including:

  • its ability to clear green water
  • damaging these algae cells and killing them.

Continuous UV light exposure will ensure that all the algae on your pond die.

The UV light works best when you combine it with a water pump and filter. When UV light kills the algae in your pond, the remains can pollute your water.

The water pump will assist you in carrying out partial water changes. About 50% water change in your pond after killing the algae can ensure that your pond remains green.

YouTube video

Will UV Sterilizer Kill Algae?

Yes. UV sterilizer will kill algae as much as it does with other microorganisms.

The UV sterilizer works by:

  • preventing the reproduction
  • damaging  nucleic acid sequences.

This destruction affects vital processes that are necessary to ensure their survival.

The overall effect of the UV sterilizer kills the organism directly or indirectly. These organisms are free-floating; hence, they cause the pond water to be turbid. Killing these free-floating algae will mitigate turbidity and enhance clarity.

How Fast Does UV Light Kill Algae?

The UV sterilizer starts working immediately you introduce it to the tank, and the organism gets exposed. It needs enough time:

  • usually up to 48 hours kills billions of algae cells in a green pond.
  • the more you expose the pond to UV light, the stronger its killing power.
  • UV light has a killing dose of algae between 22,000 and 30,000 microwatt seconds per square kilometer.

It doesn’t even take up to a microsecond to kill thousands of these organisms.

Do UV Sterilizers Kill Copepods?

No. Copepods are one of the most common zooplankton in the pond. UV light will not kill your copepods population. The pond pods are relatively safe from the effect of UV sterilizers because they have significant protective elements.

Although UV sterilizers can affect other living organisms in the pond aside from algae, it doesn’t kill some bacteria. Besides, it will require an intense exposure of zooplankton to UV light before it can significantly affect this aquarium plant.

Also, there is no need to worry because the intensity of UV light required is not enough to kill zooplanktons. So, a UV sterilizer is highly unlikely to kill any zooplankton.  The copepods will only be at risk when you expose them to a very high-intensity UV light.

How Long Does UV Sterilizer Take To Work?

UV sterilizer will start working almost immediately after introducing it to your pond. But it will take up to 24 hours before you begin to see the physical effect of the sterilizer. Sometimes, it takes up to 48 hours to complete its work if you use it to clear green ponds.

A UV sterilizer is quite effective for various purposes in its pond, and it is beneficial for clearing green pond water and killing some potentially harmful organisms. The UV sterilizer begins work once you expose it to your pond.

How Do You Tell If UV Sterilizer Is Working?

It might be slightly tricky to tell the instant impact of UV sterilizer on your pond water.  Regardless, it is essential to figure out a way to follow up on its working process.

  • You can tell if a UV sterilizer is working by using a dosimeter.
  • The UV light unit is measured in Milijoules per square centimeter.

This measurement indicates the amount of energy and requires space to deliver these rays. So, the UV light’s intensity and energy from your UV sterilizer must match the required amount for killing algae.

Should I Run My UV Sterilizer All The Time?

Yes. You might need to run your sterilizer all the time for the duration you need it in the pond. It should be on and working for all 24 hours a day to boost its effectiveness.

Switching off the UV sterilizer at any interval can permit algae growth. Remember that the growth and reproduction of algae occur within few minutes. So, if there are still some living algae, they will reproduce in large numbers during this time.

Why Is My Pond Water Still Green With UV Light?

Yes, don’t be surprised if your pond still retains its green color after using UV light. The UV light from the sterilizer will only kill the algae. It will neither remove the color of these organisms nor evacuate their remains.

So, you would still expect the color to persist for a while. The remains of the dead algae keep flowing on the pond’s surface. Although the color will not be as green as it used to be, it will still be apparent.

If you want to remove the green color of the pond water, you might need:

  • a filter 
  • a water pump

The filter will help you to trap and evacuate algae remains. And the pump will also help you carry out up to 50% water change.

YouTube video

Will A UV Sterilizer Kill Beneficial Bacteria?

No. Thankfully, the UV sterilizer does not tamper with the beneficial bacteria in the pond.

The target organisms in the pond (mainly algae) are free-floating. So, they move with water over the UV light, where it targets and attacks them. This method does not usually affect the most beneficial bacterial because they don’t move around in the pond.

The most helpful pond bacteria are in the:

  • gravel
  • wood
  • rocks
  • other substrates

As a result, they don’t get to flow over the UV light.

Does UV Light Kill Nitrifying Bacteria?

No. Nitrifying bacteria are also microorganisms, and they are subjected to the effect of UV light in the tank. In fact, this group of bacteria is photo-sensitive, which makes them more vulnerable. But the good news is that UV light will not kill nitrifying bacteria in most cases.

First off, nitrifying bacteria are photo-sensitive when they are free moving. But this light poses little or no threat when they colonize a surface. This colonization allows them to develop a certain level of resistance to light.

Nitrifying bacteria mostly colonize the filter and other surfaces in the tank. So, they rarely move around with the pond water. Remember that UV light will only kill organisms that pass over in the water column. It rarely affects none floating or colonizing organisms.

Can Too Much UV Light Kill Fish?

No. UV light cannot get too much for fish; hence, it can’t kill them. In fact, UV light does not even kill organisms attached to your fish. So, UV light is relatively safe and won’t get too much to kill your fish.

Unlike anti-fungi, anti-bacteria, and other chemicals, UV has no residual effects. It does not leave harmful chemicals or create an unhealthy by-product. Besides, UV light’s effect is more significant on microorganisms like:

  • bacteria
  • fungi
  • algae

The only way the UV light will affect the fish is by influencing other factors that might kill the fish. For instance, using the UV sterilizer during the cycling period protects beneficial organisms.

The effect of UV on some medications can cause breakage in its chelating agent’s bonds. This action can spike the amount of copper in the pond, lethal to fish.

Do UV Clarifiers Really Work?

Yes. UV clarifiers work by moving tank or pond water through a tube close to the UV bulb. These UV clarifiers aim to kill the unicellular organisms in the water that passes through this bulb.

The only cases that UV clarifiers might not work are string algae and other attached organisms. String algae do not move around in the tank. So, it does not flow through the tube that exposes the organism to the UV bulb.

  • They should kill most (if not all) algae in the tank within 24 to 48 hours. 
  • Pond clarity can take up to two weeks.

You can shorten this duration and obtain a speedy result by using a filter and a pump. The pump helps to carry out partial water changes while the filter helps trap and eliminate dead algae.

Does UV Sterilizer Kill Ammonia?

No. Ammonia is a chemical that can accumulate in the pond/tank due to the accumulation of organic waste.  The UV light can kill microorganisms, but it will not take out or limit ammonia contents.

In some cases, UV sterilizers can work in favor of ammonia accumulation. It does this by killing some beneficial organisms that can break down ammonia into non-toxic compounds. It is best not to trust UV sterilizer to help you keep ammonia level in check.

How Do You Test A UV Light Sanitizer?

You can test a UV light sanitizer for both quality and effectiveness using a dosimeter. A dosimeter is a small card that responds by changing colors to a UV ray. It responds to both far and near rays up to 200-280 nm.

  1. The dosimeter can also help you tell the UV ray’s intensity and energy, and it will also indicate the area that the rays can cover. This feature is measured in Mili Joules per square centimeter.
  2. The dosimeter card will also help you estimate the type of organism it can kill the amount of energy received. The card’s center is typically yellow or pink, while the upper and lower parts also have their distinct colors.
  3. The center colors tell a lot about the amount of UV you can expose them to while in use. Note that the best intensity suitable for your pond is dependent on the type of organism you want to kill.

You can also test the UV light sterilizer by evaluating the essential features. Check for the indicator light when the sanitizer is operating. Also, it should be easy to fit the facility into your pond or aquarium.

Does UV Light Kill Mold?

Yes. UV light with short wavelengths will kill mold like most other organisms. It attacks and destroys some vital processes in the organism. It ensures that the mold will not have the ability to reproduce until they die out.

This light will penetrate the mold‘s cell. It targets nucleic acid, which is vital for reproduction and survival. This element’s destruction causes the mold to die off within an hour or two.

The effectiveness of UV light against a wide range of microorganisms makes it a good option for sterilization. Short UV wavelength can spread over an area to destroy all microorganisms. The use of UV light to get rid of mold is more effective than most manual methods.


Green water is an undesirable condition in a fish pond. It indicates an imbalance in the level of nitrates and phosphorus. It can also be a clear indication of poor water quality.

Green water is a result of too many algae in the water, and one of the best ways to get rid of them is the use of UV sanitizer. The sanitizer produces light to penetrate and destroy the organisms’ reproductive mechanism.

The use of UV light is effective and fast. It works even better with the support of a water pump and filter. Most zooplanktons and beneficial bacteria are safe with this process.


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John Brandon

John has kept fish all his life (since he was about 5). He started with keeping guppies and fell in love with fish keeping almost straight away. That was 40 odd years ago. These days John still keeps fish and currently has two large tanks where he keeps many different types of fish such as Angelfish, Neon Tetras, Goldfish, Guppies and many more.