What Does a UV Sterilizer Do in a Freshwater Aquarium?

A UV sterilizer in your fish tank will ensure that everything goes well inside your aquarium, and especially that the entire setup looks good.

So whether you’re keeping an aquarium as passion for the aquatic life or as a beautiful, lively decoration for your home, a UV sterilizer will help a ton.

But let’s see what exactly does a UV sterilizer do in a freshwater aquarium and how to get the most out of it.

A UV sterilizer will make sure that the water in your aquarium is crystal clear by keeping algae growth under control. It helps sterilize water through UV light, destroying unwanted algae and bacteria, but keeping your fish safe.

We’ll get a bit more in depth below if you want to get behind the way an UV sterilizer works. This way, you can decide if you should get one or not.

uv light

Based on personal experience, I would say that an UV sterilizer is not a must for aquarists, but it is a device really useful for beginners as it helps keep the water clean without much work on your side.

This way, you have lower chances of getting cloudy water or keeping algae bloom under control, including excessive growth of green algae in your aquarium.

The best choice would be an UV water filter like this one available on Amazon (affiliate link).

It is built in such a way that it keeps the water moving (like a filter) and the light itself is not visible like with most models – which is always a bonus. Plus, it’s the #1 Amazon bestseller at the moment of writing, so it’s clearly a highly regarded product.

Now, if you want to get a bit more in depth with the entire process, keep reading to learn what a UV sterilizer does in a freshwater aquarium, how it works, if you really need one for your own fish tank and what you can do to maximize its efficiency.

How does a UV sterilizer work in a fish tank?

A UV sterilizer is a filtering tool, specifically designed with a fluorescent bulb that emits UV light.

This light has a germicidal effect, basically denying the microorganisms that float around the bulb (such as algae or bacteria) the option to reproduce and multiply.

As a result, the UV sterilizer either kills these microorganisms, or at the very least shortens their lives.

UV aquarium life

So, as a short answer to this question, a UV sterilizer works as a highly efficient filtration system, capable of killing algae, spores, parasites and other non-beneficial bacteria and microorganisms.

In a dirty tank, you will actually start to see visible improvements of the water clarity and quality as soon as a few days after installing it.

What does a UV sterilizer do in a freshwater aquarium?

Now, how does that impact an aquarium? Well, obviously, when the tool is placed inside the aquarium, the water will naturally flow through its UV system, exposing every particle to the UV light and, therefore, to the sterilization effect.

Is the UV sterilizer harmful for the fish tanks and other healthy bacteria inhabiting the aquarium? Is it going to stop the reproduction of the fish, or to kill your beloved life forms from inside the aquarium? Is it going to affect your plants ? Nope. Definitely not.

The UV sterilizer is not only perfectly safe for your fish tanks, but also a protective wave that keeps enhancing their life quality.

The only particles targeted by the UV-C rays are those free floating microorganisms within the water, neither attached to fish, corals or your beautiful plants, nor attached to the surface of the aquarium.

The germicidal effect of a UV sterilizer is simply going to prevent from the development of algae and other harmful bacteria that may develop in your aquarium due to the nutrients and light that the freshwater is continuously exposed to.

What you’ll get will be a much clearer water, a cleaner aquarium, and a healthier, algae-free, parasites-free, ich-free environment for your plants and fish.

How to maximize the efficiency of a UV sterilizer in a freshwater aquarium?

Now that we’ve already decided how important – and not only safe, but actually protective and useful – a UV sterilizer is in a freshwater aquarium, let’s see what influences its efficiency in filtrating the water accordingly, and how we can increase its efficiency to the maximum.

uv sterilizer faq

Here are the main factors influencing the effectiveness of a UV sterilizer and how to improve them:

1. Bulb Age

The younger the bulb, the more efficient the sterilization process. As the light bulb gets older, its radiation and capacity to kill non-beneficial microorganisms may reduce by 40-60%.

So, it’s essential to read the manufacturer’s recommendations for when you should change the bulb, in order to make sure that you’re getting the best effects.

Some light bulbs are recommended to be replaced after 6 months, whereas other may work at the same efficiency up to 14 months.

2. Bulb Length

The longer the bulb, the longer the contact time with the water and, consequently, the more efficiently it will work, as it will have more time to kill the parasites, algae or residues.

You don’t have to go over the top and get an extremely long bulb, though, that’s as long as the tank itself.

For example, an UV light bulb with the length of around 5 – 7 inches can keep a 30 gallon tank clean without a problem.

3. Aquarium size

You should make sure that the UV sterilizer is capable to offer protection for the size of your aquarium.

Larger aquariums need a larger sterilizer in order to ensure the coverage of the entire area.

At the same time, a large UV sterilizer in a small aquarium may be overkill, take up too much space and even warm up the water too much, with potential deadly effects to all living beings inside.

In order to choose the right size for your setup, make sure to check out the specifications on the model you’re planning to purchase.

4. UV Penetration

The UV light emitted by the sterilizer will need to be able to penetrate the water in order to have an effect. There are various things that could prevent that from happening.

The most common is getting the bulb dirty: any mineral or goop build-up on the bulb will result in it being less and less effective. Therefore, cleaning it regularly is required (make sure to turn it off before touching it!).

Other things that could interfere with the UV penetration are turbid water, salted water and stationary water.

Since the UV light will only filter the water it gets in contact with, it’s best to have an active filter that continuously moves the water in your aquarium.

Or get a model like the one I recommended above that comes with a built-in filter to solve this problem.

Do you really need a UV sterilizer for your aquarium?

clean aquarium scape

While an UV sterilizer itself is not enough to keep an aquarium safe and clean and is not a replacement for an actual filter, it is a useful additional tool that helps keep your water clean and your fish happy.

You can check out my recommended aquarium filters here.

Most aquarists I know can run an aquarium perfectly fine without an UV sterilizer (myself included), but if you are just starting up, you will get rid of many headaches if you install one.

It makes everything easier and ensures that the crystal clear water you see in your tank on day one will be there on day 30 – and not a mucky, greenish water instead.

Related reading: How to get rid of white film on top of aquarium water.

But, again, you have to make sure that you choose a UV sterilizer with a size that fits the size of your aquarium.

Smaller aquariums require smaller sterilizers for the filtering of the freshwater, whereas larger ones need larger and stronger UV sterilizers for full coverage.

My favorite is the Codia one (affiliate link) as it keeps the light hidden unlike most models (and does a tremendous job), but you can choose any that you like because they all work the same.


The main advantage of using a UV sterilizer in a freshwater aquarium is the additional filtering and sterilization of the freshwater, without any of the side-effects of using chemicals.

It is perfectly safe and non-invasive for the life forms you are growing and highly effective, working well both from day one when you have no problem, but also after an algae bloom or anything similar.

Although in the second case, it will need some time – up to a couple of weeks – to get you rid of the problem and might need additional help from your side (water changes, for example).

But, even so, keep in mind that you should never count on a UV sterilizer as a main filtration system.

It is a perfect addition to mechanical of biological filtering, but never the only type of filtering that you should use, because it only impacts the microorganisms floating freely through the water that goes through the sterilizer itself, so it needs the water to be constantly moving in order to work.

UV Sterilizer FAQ

1. Is the UV light dangerous for the fish tanks?

No, the UV light will not kill or harm your fish, snails, shrimp or plants. In fact, due to cleaning the freshwater and killing the parasites, spores and non-beneficial bacteria, it will actually prevent fish tank death and will protect your fish from diseases such as ich and many others.

While ultra-violet light may be dangerous and even cancerous for people, with a sterilizer correctly installed and used, in an aquarium with appropriate size, everything is safe and protected.

And this is another reason to go for an enclosed one like the model I recommended above.

2. Will the UV sterilizer harm the healthy bacteria in the aquarium?

No, unless that healthy bacteria flows in the water and through the sterilizer – which it usually doesn’t.

The beneficial bacteria from aquariums usually stay on rocks, on wood and decorations or on the gravel on the bottom of your aquarium.

The UV light will only impact the microorganisms floating through the water, not attached to any surface or fish.

3. Can a UV sterilizer save sick fish tanks?

Unfortunately, not. The UV sterilizer only works as a filtering system that will result in crystal-clear water, perfect for a healthy life for your fish tanks. But it is by no means a treatment system for already sick fish.

And that is because, when the fish are already sick, it means that the harmful bacteria or parasites are already attached to the fish, not floating through the water.

So, filtering and sterilizing the water with UV power will not be enough to save your fish. You will need to use specific medication in order to save your fish tank.

But if you’re only dealing with mucky water – green water or an algae bloom – that is something an UV sterilizer might be able to fix as long as you take additional measures like changing the water and manually cleaning the aquarium’s glass and decorations.

If you still have questions about UV filters, don’t hesitate to let me know by commenting below.

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