How Long To Leave UV Light On In Aquarium – A Guide For Beginners

Taking care of your fish tank requires the support of multiple tools, one of which is a UV sterilizer. However, not everyone knows how to utilize UV lights the right way.

If you are thinking “How long to leave UV light on in the aquarium?” then the answer will be addressed right below. Keep reading to see for yourself!

How Long To Leave UV Light On In Aquarium?

It is best to leave UV light on for roughly 10 to 12 hours per day. This can help establish a fixed routine for your fish and give them time to rest.

Remember, if you do not intend to expose your fish to sunlight 24 hours per day, there is no reason why UV light should be any different. Having UV light on for too many hours can be detrimental to your fish’s growth.

What Is UV Light?

UV light

UV light – also known as UV sterilizers – is a tool employed in a fish tank’s filtration system. Usually, it is placed at the end of the filtration line, right after the water filter. It usually comes in the shape of a tube, which emits ultraviolet light.

Why Do You Need A UV Light?

If used correctly, UV light can bring about significant benefits for your fish tank.

First off, it helps sterilize and clean the water effectively. UV light can kill off ich, bacteria, and parasites in no time. Furthermore, it also minimizes the growth of algae and prevents it from overtaking the entire tank.

Secondly, UV light can filter the water and ensure it has a crystal-clear color. This is closely linked to its ability to kill single-cell organisms as mentioned above.

Once all of the unwanted living beings are gotten rid of, your tank’s water is sure to be pristine.

How To Choose The Best UV Light For Your Aquarium

Choosing the best UV light for your aquarium is no easy task, as the market is filled with different products. Below are some of the most important factors you should pay attention to when shopping for UV light.

Types of UV light

There are two common types of UV lights. The first one is stand-alone light, which has its water flow. It can be placed directly inside your tank without attaching to any other technical device.

The second one is inline-connected UV light. For this to work, you will need to connect it to your canister filter. Or, you will have to connect the tube to a separate electrical source using its powerhead.

Tank size

Different tank sizes have different needs when it comes to lighting. The bigger your tank is, the more UV light it will need. If you are not sure about this, check the label on each UV light package.

In most cases, the manufacturers will indicate the best tank size for all products. Check the one that best fits your tank and you are good to go!


Powerful UV is more than capable of eradicating harmful organisms inside the tank. If you want to keep the water pristine, make sure to invest in UV light with a high capacity.

If all you need your UV light to do is to act as a safety net, a small one would suffice.

Additional features

Some UV lights come with more features than others. For example, there are UV bulbs that can be used for both freshwater and saltwater tanks. Or, you may have self-cleaning UV light that will function on its own.

Depending on your needs, feel free to pick a product that comes with smaller details like these.

Price tag

UV lights vary significantly in their cost. Some items may cost as little as 20$, while others can go up to more than $500. While you do not have to strive for products you cannot afford, it is wise to stay away from anything too cheap.

Such UV lights are not safe for your tank and may end up breaking inside the tank.

Aquarium UV Light Maintenance

Some people are concerned about maintaining the UV light system. However, it is not too different from cleaning other filtration systems. Every once in a while, disconnect the light from its electrical source.

Next up, clean the tube and make sure nothing gets stuck inside. You may also use a washcloth with a mild cleaning solution to minimize the impact of chemical substances on such delicate material.


1. Does UV light kill beneficial bacteria?

No. Beneficial bacteria mostly stay on gravel, driftwood, and rocks only. Since they do not float in the water, their chances of being killed off are quite low.

That said, this does not mean that UV lights cannot tamper with the growth of such useful bacteria. Hence, if you want to minimize the effect of UV light on beneficial bacteria, make sure to keep the installation away from most surfaces.

2. When should I turn off the UV light?

tropical fish under UV light

There are some occasions when you should turn off the UV light. These include when you first set up your tank, as bacteria need to establish a nitrogen cycle before being killed off. Furthermore, you should also turn off your UV light whenever you put in beneficial bacteria.

Last but not least, if you medicate sick fish or treat the water using chemical products, consider turning off the UV light so that its effect does not tamper with other processes.


How long to leave UV light on in the aquarium should no longer be your concern now. As long as you follow the instructions above carefully, your fish tank is sure to benefit from such a useful item.