How Long Do You Leave A UV Light On In An Aquarium

One aquarium item that we have not talked much about is the light source. Lighting in your tank is important. You want your tank to resemble the natural environment of your fish.

So the light in the tank would act as the sun. With that being said; since the sun doesn’t shine for 24 hours, you do not want the light in your tank to be on for 24 hours. Your fish need time to rest and re-energize, and they do this when the tank lights are turned off.

How Long Do You Leave A UV Light On? In order to give your fish and your live plants the lighting that they require, you should only have your aquarium lights on for 10-12 hours a day. In order to make sure your lights are on for this optimum amount of time, you can install a timer. 

You can also purchase a lighting kit that has a built-in timer. This way, if you are unexpectedly gone, you do not have to worry about your fish. 

There are 2 basic types of lighting for your fish tank. Your ordinary lighting system is similar to the lighting in your house (though LED is becoming popular too), and there is UV lighting. Today I am going to talk mostly about UV lighting. I will cover the most important and prominent information on Ultraviolet light.

Ultraviolet Lights For Aquarium

The UV light is also called a UV sterilizer. I will refer to them as both, throughout the article. 

It was given this name because it helps to sterilize your fish tank. It will rid the tank of algae, bacteria, ich, and other parasites. You can look at it as a “safety net” for your fish tank. UV lights also help in keeping your tank water crystal clear.

The UV sterilizer is a tube of ultraviolet light that will kill the single-cell organisms by damaging the DNA. Despite rumors and other things you may read on the internet, as long as the UV light is used correctly and cautiously; it is NOT harmful to you, your fish, or any invertebrates you have in your tank.

UV Light For Aquarium Types and Shapes

The cost of the UV light is a little more than what you normally use in a fish tank. It will be about $35 but can run into the hundreds depending on the brand, the water flow, and the power.

Added features will make the cost rise as well. The type of UV sterilizer you choose will definitely depend on the water feature in your tank.

The design of the UV light is normally a device that looks like a tube that will have an in-and-out flow. There are 2 types of UV lights:

  • Stand alone: this type will have its own water flow
  • Inline connected: this one will have a powerhead, or it can be connected with the outflow of your canister filter

It is suggested that if you are going to use it on a canister filter it has to be hooked up to the outflow in order to kill the algae, bacteria, and parasites. 

The UV sterilizers will come with a UV bulb. Most ultraviolet bulbs will last 12-14 months. Your skin can actually damage the UV bulb so it is highly recommended that you wear gloves when you handle them.

Aquarium UV Light Efficiency

Ultraviolet lights can be extremely efficient if used properly. It is of the utmost importance to match the water flow from the filter to the time of exposure on the UV bulb. For example: if the water flow is too strong the water exposure to the UV bulb will be short.

This means it will not be efficient enough to kill the microorganisms. Just like, if the water flow is not strong enough, then it will take a lot longer for the microorganisms to die because they will have to go through the water flow 2 or 3 times to effectively kill the DNA.

Make sure you are matching the ultraviolet bulb power to the water flow power. It is highly recommended to install the UV light after you have ‘cycled’ your tank.

Does UV Light Kill Beneficial Bacteria?

UV Light Will Not Harm The Beneficial Bacteria. Your beneficial bacteria grows on rocks, wood, gravel, etc. So unless it is scraped off of one of these surfaces, it will not be floating in your aquarium. The only way your beneficial bacteria can be killed is if some of it flows in the water and goes through the UV sterilizer.

You want to read the directions or manual very carefully before you begin. DO NOT plug anything in, for any reason, until it is all installed. It is best to unplug everything from your aquarium while you install the UV light. 

The UV light is safe for plants, fish, coral, and invertebrates. There are different ideas and opinions all over the internet, but rest assured, it will NOT harm your fish. Humans should not look directly at a UV light, so make sure everything is installed and ready to go before you actually turn it on.

How To Choose The Best UV Light For An Aquarium

As stated above, there are various types and styles of UV lights on the market. Most are very simple and just hang on the side of the tank. You can also get UV lights that are submersible if you prefer that style.

Two of the most important factors to consider is the size of your aquarium and the power of the UV bulb! 

The size of your aquarium: All manufacturers will indicate, on the packaging, the aquarium size that is best suited for the UV light. There are some types of UV lights that are built into your filtration system and there are others that you can easily connect to your filter. This can save you a little bit of money because you will only need to have 1 pump in the tank. 

The power of the UV bulb: The more powerful the bulb the easier it will be to kill all of the microorganisms in the tank. The weaker a UV bulb is the longer it will take and it may even struggle with some of the organisms.

There are also some additional features, depending on how much money you are willing to spend on your UV light. One example of an additional feature; the UV sterilizer being set up to work in both a saltwater and freshwater tank. Some models will only work in one or the other. 

How To Care for the Aquarium UV Light

The maintenance of a UV light is no more difficult or different than that of your aquarium filtration system. As long as you keep the pump clean, it will not get clogged and your UV light will work just fine.  But, you will also need to lean on the crystal sleeve. When the sleeve is dirty it is hard for the UV bulb to penetrate through the dirt and to do its job properly.

When using a UV light, your regularly scheduled water changes are very important. The UV light kills the algae, but it remains floating in the water. Daily cleaning with a net to remove the dead algae is suggested. This will help to keep your water from turning green between your water changes.


Most of what we know about UV lights/sterilizers are from consumers who have purchased and used them. There are different opinions on how long you should leave them on. Some consumers say 10-12 hours, to mimic the natural habitat of the fish, while others say that you should leave them on for 24 hours.

The manufacturer will have the suggested time frame with the instructions, and that would be my recommendation. One constant, with all of the research and reviews that I read through; a UV light will clear your tank water in up to 2 weeks and they help to keep your aquarium healthier for your fish.

They do this by killing all of the bad things in your tank; algae, parasites, bacteria, and more.

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I'm passionate about fish pets and love sharing everything I learn about them.

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