How Long To Leave the UV Light On in an Aquarium

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by Jason Matthews



The UV light in your aquarium is not the same as the light you use to see your fish and light the tank. It is an important part of your tank’s ecosystem that helps to clean and sterilize your tank.

Unlike a regular aquarium light, you can run a UV sterilizing light 24 hours a day. The UV light in your tank is not like a white light. It is part of the tank’s filtration system and is intended to remove microorganisms. 

Read on to learn why UV light is important and how long to leave it on for optimal results.

What Is a UV Sterilizer?

A UV light might also be referred to as a “UV sterilizer.” These terms mean the same thing and can be used interchangeably. A UV sterilizer is part of your filtration system. Place it at the end of the filtration line, after your water filter.

UV sterilizers are safe for the fish in your tank because only organisms that pass through the filtration system are affected. Swimming pools, professional aquariums, and many other settings use UV lights for sterilization. They are not harmful to fish.

How Does a UV Sterilizer Work?

How Does a UV Sterilizer Work in aquarium

When water passes through the UV light, the ultraviolet rays can kill single-celled organisms and other microorganisms, sterilizing the water. The UV light mutates the organisms’ DNA, making them unable to reproduce. UV light can therefore save your aquarium fish from potentially deadly infectious diseases.

Different wavelengths of UV light target different types of microorganisms. UV lights can kill many organisms, from bacteria and viruses to algae. Here is a breakdown of which types of UV light kill which organisms:

  • 15K microwatt seconds per cm2 — bacteria
  • 15K–30K microwatt seconds per cm2 — viruses
  • 22K–30K microwatt seconds per cm2 — algae
  • 45K microwatt seconds per cm2 — fungi
  • 90K microwatt seconds per cm2 — protozoa

An expert at your local aquarium shop should be able to help you choose the right kind of UV light for your tank.

How Long To Leave the UV Light On in an Aquarium

UV Sterilizer fish tank

The UV sterilizer in your tank is not the same as a regular light. It is better to leave it on 24/7.

Make sure you buy the right size and strength of sterilizer for your tank. If the sterilizer is too small or too weak, it will not expose the microorganisms to enough light, or will not expose them for long enough. In this case, the UV light will not be sufficient to kill them.

When you stop running the UV light, parasites and bacteria have a chance to build up in the tank. Although some people recommend turning the light off occasionally to allow zooplankton and phytoplankton to build up, we don’t recommend it if your main goal in having a UV light is to keep the tank free from microorganisms.

Related article: How to apply a aquarium background

Other Types of Aquarium Lights

You may also have a 7000K watt white light in your aquarium or some LEDs. Or your tank might be near a window or another light source. If that is the case, these lights need to be monitored and turned off regularly.

These types of light affect fishes’ circadian rhythms. When they are left on all the time, fish do not have a chance to sleep and re-energize. Unlike UV sterilizers, you should only leave these types of light on for a maximum of 10–12 hours per day.

When To Turn Off the UV Light in an Aquarium

There are a few circumstances under which you should turn off the UV light in your aquarium. These are when you are setting up the tank, adding beneficial bacteria, or using a medication that stipulates that the UV light should be off.

Setting Up the Aquarium

When you first set up your tank, it requires a couple of weeks for the various cycles of the ecosystem to establish. At this time you want bacteria to grow and establish a nitrogen cycle.

Leave the UV light off when you first set up the tank so that good bacteria colonies get a chance to start doing their job of breaking down fish waste.

Adding Bacteria

Similarly, you should turn the UV light off whenever you seed beneficial bacteria. You might need to do this if you have accidentally depleted your bacteria during cleaning or if you had to do a large water change to take care of a problem.

Keep the UV light off whenever you add new bacteria to the tank to allow the colonies time to stabilize.

Using Medication or Water Treatments

Some medication or water treatments stipulate that the UV sterilizer should be off before adding the treatment to your tank. Read the directions for anything you add to your tank and follow them closely.

How To Maintain a UV Light

If you leave your UV light on 24/7 you will need to pay special attention to maintenance. You need to replace the UV bulb every 6-12 months, depending on the type of bulb and the amount of use it gets.

Keeping a record of tank maintenance is a good idea for all types of routine tasks like cleaning, water changes, etc. When you install your UV light, make a note in your maintenance record so you can track when you need to replace the tube.

The UV bulb’s strength is a key component in how much radiation a UV sterilizer produces. A UV light that is performing sub-optimally will not produce enough radiation to keep your tank free from bacteria and parasites. 

You should also pay attention to the other components in your filtration system to ensure optimal performance. If your pump pushes too much water through the sterilizer, the UV rays will not have enough time to kill the microorganisms.

When used correctly, a UV sterilizer is an excellent addition to a water filtration system. Leave it on and check it often to keep your tank water crystal clear and healthy for your fish.

Related: How To Lower TDS in Aquarium


Jason Matthews

My name is Jason Matthews, and welcome to my website. When other kids were bragging about how their dog could sit and roll over, I was bragging about my latest Betta Fish and the cool sea castle I just added to his aquarium. 

Jason aquariume

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