Are Black Lights Bad For Betta Fish?- Should I Avoid It?

Are Betta Fish Smart

Black lights are becoming more and more popular in aquarium decoration. The main purpose is to make the fish ‘glow” in the dark. Sounds cool until you notice your betta’s health deteriorating. Is the black light not safe for bettas?

Black lights radiate Ultraviolet wavelength into the fish tank. It is claimed to be harmless by the manufacturers. But prolonged exposure to UV rays can cause irritation in the eye, skin damage, and a saggy or wrinkled body in your betta.

You don’t have to restrict yourself from all the fun if you know how a black light affects bettas and when it’s time to turn it off. And we are here to make sure you know everything about using black lights in a betta tank. Keep reading if you don’t want to hurt your little pet accidentally.

Key Takeaways

  • Blacklight or UV light is not harmful as long as it produces UV-A rays.
  • UV-B ray (250-315 nm) is used in mineral hunting black lights.
  • Exposure to UV-B rays can hurt the Betta’s eye and can cause skin damage.
  • The betta will glow better if it was genetically enhanced during earlier life stages.
  • The safest black light in the market should produce a wavelength close to 375 to 380 nm.
  • UV wavelengths are shorter than visible light, making them invisible to the human eye.

What’s The Purpose Of A Black Light?

Black lights are basically no light at all. The tool generates ultraviolet radiation when activated. That’s it. There’s no fancy mechanism behind the term.

The interesting part is we can’t see ultraviolet rays or UV radiation. The spectrum is too narrow to be captured by human eyes. However, once the UV light hits certain objects, the radiation gets absorbed. Later, the radiation is reflected with slight moderation. If you are a viewer, you now get to see the object even though it’s completely dark.

Hence, it looks like the object is glowing. That’s why aquarists attach a black light on top of their aquarium lid. It lets them watch over their fish’s activities even after the lights are out. Not to mention, the scene is ethereal. Colorful bettas glowing in a glass tank is something you have to see at least once.

Usually, when the object has an element called “phosphor,” it manipulates the invisible UV light and makes it visible to us.  To be able to see your fish glow in the dark, the fish must have the right patterns on its body. Thankfully, most Bettas flaunt vibrant stripes on their bodies. So, the black lights will work on a Betta and make it glow in the dark. How exciting, isn’t it?

Are Black Lights Bad For Betta Fish?

The main reason people want black lights is for fun. It just looks nice to have your aquarium glow in pitch-black darkness. After all, bettas are supposed to stay without light for at least 10 hours. that’s a lot of time without seeing what that little buddy is doing.

See also  Do Betta Fish Need Light?

But the question is whether it’s bad or detrimental to a Betta’s health in any way. The answer is both yes and no. If you choose the right type of UV light and know how much is too much, you won’t face any trouble. However, there’s a huge chance you will mess up if you are new to this hobby. In that case, the poor fish will have to pay the price.

No matter what the manufacturers say, a UV light depends on UV radiation. And UV wavelengths are shorter than our visible light spectrum. Human eyes can only see light containing wavelengths from 700 nm to 380 nm. It is termed ultraviolet once the wavelength is smaller than 380nm (violet). From 100 to 380 nm (arguably up to 400 nm) falls within the UV category.

Name Of RadiationApproximate WavelengthRemarks
UV A315-380 (or, 400)Comparatively safer (used in black lights)
UV B250-315Damaging; but safer than UV C
UV C100-280Extremely damaging

 As you can see, both UV B and UV C are dangerous for the skin. That means shorter wavelengths are more damaging. That’s why a safe black light must have wavelengths limited to UV-A. Some people don’t even consider UV-A fully safe. That’s why reliable manufacturers stick with 375 to 380 nm, which is the weakest form of UV light.

If you try to increase the wavelength further, it might enter into the visible spectrum. It won’t be a black light anymore. So, you have to make sure the wavelength is not “too short” before purchasing. Even after you take care of all these issues, there is a chance the light won’t suit your betta. After all, some bettas are more sensitive than others.

The most common symptom of irritation due to UV light is a disrupted sleep pattern. Your little buddy will find it hard to sleep at the usual time. The presence of light doesn’t let the fish get out of is guarded mode and rest.

Next, if you keep using black lights for a while, the vibrant color on your betta’s body will start to fade away. It generally means the skin is aging at a rapid speed. That’s why humans apply UV protection creams while stepping out (to protect against skin damage). 

Any further usage will cause severe skin damage and ultimate death. So, you see, there are lots of risks associated with these trendy lights.

4 Things To Consider Before Getting A Black Light For My Tank

If you have already decided to go and buy a black light, it’s too late to consider why you even need one. To each his own. However, you must remember some things while dealing with a black light. Have a look.

1. Don’t Use For Too Long

The key is knowing when to turn it off. Usually, experts advise us to provide at least 12 to 14 hours of ‘daytime.” To imitate natural daylight, you can use LED or fluorescent light bulbs. During nighttime, you can turn the black lights on. The period would be around 8 hours every day.

To make the black lights perform better, the room has to be completely dark. Don’t make the mistake of using black light as the primary lighting source. Too much exposure to UV rays (even if considered safe) can negatively affect the betta.

See also  What Color Light Do Betta Fish Like? – Warm Or Cool?

2. Choose UV-A Radiation

You have to choose the least harmful UV rays available in the market. It shouldn’t be too bright. Even though the range for UV-A wavelengths starts from 315 nm, try to stay close to the other end. The weaker it is, the better. Pay close attention to how your fish behaves once the light is activated.

3. Use It For Sterilization Purposes

It is perfectly safe to use UV lights to sterilize the fish tank. UV wavelengths are powerful enough to kill harmful bacteria that are not filtered. Usually, UV sterilizers radiate 250-253 nm of wavelength into the tank. The purpose is not to see a glowing fish. It is an easy way to make sure your Betta is not swimming in germ-filled water. But the problem is its wavelength.

To be effective, most UV sterilizers generate UV-B rays. But UV-B ray has the potential to severely harm your betta’s skin. That’s why you will need to remove the betta from the tank first. Then, sterilize the tank using a black light or UV light. Usually, UV sterilizers are okay to be put inside the water. Since we are talking about black light, just hang it over the aquarium for a while. Then, transfer the fish back into the aquarium once the sterilization is over.  

4. Get The Genetically Enhanced Bettas

Some fish are genetically enhanced during the infant stage. A special protein resulting in fluorescence is added to the fish’s body. It happens to potential fish, including bettas, tetras, and so on. Thanks to this special element, the fish glows exponentially.

You can ask for such modified bettas in the pet store. If the Betta never went through this procedure, it wouldn’t glow as much under the UV light. Like every other item in the tank, including green plants, rocks etc., the fish will also stay in the dark.

cornflower blue betta

Can Black Light Affect My Betta’s Eye?

Some users complain that black lights hurt the betta’s eyes. As we have said earlier, not every betta is going to respond the same way. Some fish can tolerate more. Also, did you know that bettas don’t have eyelids? It prevents them from closing their eyes. That’s why a slight change in the lighting setup can hurt their eyes badly.

Because Bettas can’t see the UV-A radiation, we can see that. And, the constant exposure to such lighting can irritate their sensitive eyes. We highly suggest you only use black lights after the fish is asleep. Don’t stretch the UV-light period for more than a couple of hours.

Another reason why Bettas can react negatively is due to using the wrong type of UV wavelength. Some black lights are meant for mineral hunting. The task requires a SW UV light producing wavelengths near 255 nm. If you have paid attention to the table earlier, you will know what’s wrong. It’s not UV A anymore. 255 to 315 nm of wavelengths fall under the UV-B category.

Not only that, 255 nm is quite close to the edge of the UV-B spectrum. Only 5 nm is left before it enters the UV-C spectrum. You must check whether the black light is specifically made for aquarium or not. If you have got the “mineral hunting” one, it explains all the problems you have been facing. Remove it as soon as possible.

See also  Why Is My Betta at The Bottom of The Tank? [11 Reasons]

Do Black Lights Need To Be Bright?

You don’t want the black light to be too bright. How can a black light be even brighter? The reflection you see through the black light should be “too much.” Less is more when it comes to using black lights. Don’t fall for the packaging claiming 10W or 20W of power generation.

The problem is that the term “wattage” often indicates how much electricity the product uses in a given time frame. It doesn’t mean that a 40W light must produce more light than one with 20W. It all depends on how efficient the light is. We suggest you check how much power the black light actually generates while it’s ON. If it’s 2W, then use it for tanks up to 8 to 10 gallons. That’s the general rule.

If it’s a big tank (like 50 gallons), look for something between 8W to 10W. As you can see, we are looking for lights with restricted capacity compared to normal LED white lights for the day. It’ because we don’t want the UV light to be as bright or in the face. Considering it will be turned on during your betta’s sleep time, it shouldn’t be “too loud.”

What Is A Safer Alternate To Black Light?

Have you heard of Actinic Light before? It can be used alongside black light to reduce the use of harmful UV light. Actinic light has longer wavelengths than typical black light (which radiates UV-A). the wavelengths are normally within 400 to 450 nm range. As we know, 400 nm to 500 nm falls within the visible light spectrum (blue light). That means you will be able to see the blue light illuminating your entire tank.

It won’t feel like the fish are glowing and swimming in pitch-black surroundings. However, on the sunny side, Actinic bulbs create almost the same (we would say 60%) aesthetic as a black light. The tank looks more bluish and aesthetically pleasing. Any object with vibrant color will pop out even more. In this case, the object is your little betta. The more colorful stripes it has, the more beautiful it looks under an Actinic light.

Plus, there are no harmful side effects at all. The wavelengths don’t penetrate the skin or cause any sort of damage. We think it’s an overall good deal. Instead of using black light as the main source of illumination, you can go 60-40. 60% of the time, including the whole day and evening, tun the Actinic light on. It will support the betta’s vision without hurting.

Plus, the light is clear enough for them to swim and hunt insects. Once the fish starts resting, you can switch to the black light. We believe a healthy betta wouldn’t mind tolerating the UV light for a few hours.

Before You Leave!!

Often, people assume bettas are afraid of the dark. That’s why they opt for black lights. But they can’t be more wrong. Bettas are not afraid of the dark. In fact, they need darkness for at least a few hours every day. To learn more on this topic, please follow our article on “Are betta fish afraid of darkness.”

Sharing is caring!

Muntaseer Rahman

About Author

Hello, I’m Muntaseer Rahman, the owner of I’m passionate about aquarium pets like shrimps, snails, crabs, and crayfish. I’ve created this website to share my expertise and help you provide better care for these amazing pets.


This site is owned and operated by Muntaseer Rahman. is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to This site also participates in other affiliate programs and is compensated for referring traffic and business to these companies.