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Should I use a UV Sterilizer in a Freshwater Aquarium? (Or a Marine Tank?)

UV (ultra violet) light sterilizers for aquariums are not as common as I thought they would be. I don’t personally know anyone with an aquarium that is currently using one. Many people with outdoor ponds have them. Does that mean you shouldn’t use one? Good question! Aquarium UV sterilizers certainly have their place and are very useful and effective, if they are used for the right reasons.


Should you use a UV sterilizer in a freshwater aquarium? If your aquarium water is cloudy from water borne microorganisms such as free-floating algae, then yes, you should use a UV sterilizer. If algae buildup is a concern then yes, a UV sterilizer will help keep it under control. A UV sterilizer will kill bacteria, pathogens and other water borne bacteria. A UV sterilizer will also help to clear green water syndrome. 


So, you can see there are definitely very good reasons to use a UV sterilizer. It is such a simple piece of equipment to operate and will help to promote overall health in your fish tanks. Understanding how the sterilizer works and what it can do to make your aquarium a happier and healthier environment will make your choice to get one that much easier. I have done the research for myself as I need to buy one and now will explain what I understand so you can make the choice yourself as well.

Internal UV Sterilizer with Power Head

What is the Purpose of a UV Sterilizer in an Aquarium?

UV sterilizers have a unique job in an aquarium setup. They provide a completely different result than a filter so adding a sterilizer to your tank would be an addition to the filtration not a replacement.

The purpose of a UV sterilizer is to neutralize water borne bacteria, algae and pathogens of all kinds to improve the waters health for your fish. This is not only the purpose but the benefits of using one.

The important thing to note is the “water borne” part of that last sentence. This means if you have hair algae or any other type of algae that grows without floating in the water, a UV sterilizer will not be effective on those types of things. You will have to remove the algae by cleaning and other means.

UV sterilizers are very popular for outdoor ponds but have really made an impact on the aquarium hobby over the last 20 years or so.

It is also important to note that UV sterilizers can and should be used in both freshwater and marine fish tanks, not just freshwater. 


How Do UV Sterilizers Work?

Water is drawn into the sterilizer where the UV light resides. As the water passes through the light, the DNA in the bacteria and algae mutates, preventing it from growing and neutralizing the bacteria. This prevents microorganisms from growing. This results in much cleaner and clearer water for your fish tank. Yes, it’s that simple.


What Kind of UV Sterilizers are There?

There are a few different types of sterilizers I have found.

  • Inline UV sterilizers are great for using with a canister filter and or sump tank. They are installed in the outlet portion of the hosing. This is recommended so there isn’t as much organic material flowing through the sterilizer, which could have been caught by the filtration system. Remember, you are trying to isolate and destroy waterborne bacteria and pathogens not filter out larger organic materials that a filter is designed for.

This inline UV sterilizer is an excellent option over on Amazon

  • Hang on back UV sterilizers are a nice option and if you are already using a HOB filter, then you probably have room between your fish tank and wall to hand this unit as well. These HOB UV sterilizers are basically the same setup as an HOB filter in that you hang it on the side of your tank with the inlet tube in the water. It sucks the water in to be sterilized then pours the sterile water back into the tank. The benefit to using one of these styles is the extra water flow and surface agitation it will cause.
    • It is also important to note that this style is about the easiest to figure out and install out of all 3 that I am going over.

This HOB UV sterilizer is an excellent option over on Amazon, It also has a surface skimmer. 

  • Internal/submersible UV sterilizers are easier to install than an inline unit but not as easy as the HOB style. Internal units come with a small power head and are inserted right in the water. They will add quite a bit of flow to your water as well as some extra surface agitation, which is always a bonus and beneficial to your fish and their health.

This submersible UV sterilizer is an excellent option over on Amazon

That should be enough information to make a decision, if you were undecided on whether to purchase a UV sterilizer or not before you found this article. I would recommend checking out one of the links above, as I have posted some very good options for you.

Up next, I am going to quickly answer some related questions I found online that I feel should be answered here as well to give you more information.


Is a UV Light Harmful to Fish?

Yes, if exposed to the light, your fish will be harmed. They could go blind. It’s also not wise for humans to stare at a UV light that is turned on. Luckily, all UV sterilizers are sealed units and should not be turned on until after the light bulb is installed and the unit is closed back up and ready to be used.


How Long Should a UV Light be on in an Aquarium?

From the research I have done, having them turned on for a day or two the first time you set it up is sufficient and then a few hours a day or as necessary after that. Of course, you should make sure follow all instructions on the packaging provided by the manufacturer. After using it for awhile, you will know how long you need to let it run, based on your fish tanks needs.


Can You Use a UV Sterilizer in a Planted Tank?

No, a UV sterilizer will not kill live plants in a freshwater tank. I have read on some forums where a hobbyist was saying his sterilizer killed his fish. It must be a coincidence though, as there is no way for this to happen. This goes for the plants in your tank as well. They will be fine.


Where Do You Put a UV Sterilizer in an Aquarium?

This will depend on the type you purchase. Quickly, here is where you should place each style.

  • An inline sterilizer will be set up along the outlet hose of a canister filter. This means the unit will most likely be set up below or beside the aquarium.
  • A hang on back sterilizer will be hung on the back of the tank just like an HOB filter would be set up.
  • An internal sterilizer will be placed somewhere in the tank so it’s not in the way of anything else and can potentially help the water flow in the fish tank. Probably on the back or side of the tank so it is not in your way to view the fish.


How Do I know if My UV Sterilizer is Working?

This is such an excellent question. How would you know?

Depending on the amount of water borne bacteria in your tank, it could be hard to tell or alternatively it could be easy. If you have a full-blown green algae bloom and your water is just green, then using a UV sterilizer for a few days should pretty much clear the bloom right up. This is how you know the unit is working properly.

If you get one of these units, just as an added piece of equipment to fine tune your water conditions, it might take you more time to figure out if it is working or not. You should be able to identify something that is clearer and cleaner in the tank after a week of use. If you find after a week that there is no algae blooming, then you know it is working. It’s about knowing what’s going on in your tank. One thing I can tell you is that yes it will make a difference.


Will a UV Sterilizer Clear Cloudy Water?

If the cloudiness is happening because of bacteria and or algae blooms, then yes it will clear up cloudy water. I don’t know what else would cause cloudy water aside from your tank not being done going through the nitrogen cycle. I trust that if you are seeking information on UV sterilizers, you are way beyond the stage of simply letting your water cycle through.


How Long Does it Take a UV Sterilizer to Clear Green Water?

I haven’t cleared a green tank myself, as I have been lucky not to have that issue. What I have found in my research is it typically takes a few days and up to a week to clear a tank in this kind of condition. It will also depend on the size of your sterilizer and at what rate the water flows through it. The slower the water flows through, the better chance the UV light has to neutralize living bacteria.



UV sterilizers do not replace proper filtration, they compliment filters. They have a completely different task to complete. UV sterilizers are beneficial, not only to ponds but now more and more freshwater and marine aquarium owners are adding them to their setup to improve the tanks overall health and beauty.

UV lights should never be looked at when turned on if (by chance) it is turned on when the unit is not sealed up. UV lights can damage your eyes. On that note, I hope you have decided which style will work best for your setup, as I trust you see how valuable these pieces of fish tank equipment really are.

Good luck and thanks for reading.


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