Can You Leave Blue Lights On All Night In A Fish Tank?

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One of the most common questions I get about blue lights in fish tanks is whether they can be left on all night.

This comes naturally as this type of light adds natural beauty to the tank. So, to make life easy for you, I decided to collect everything I know about this topic into one article.

Let’s dive right into it.

Can You Leave Blue Lights On All Night In A Fish Tank?

Leaving blue lights on all night in a fish tank may change your fish’s behavior, encourage algae growth, and disturb your fish’s sleep, which can cause stress in the long run. It is generally not recommended to leave aquarium lights on 24/7.

This question matters to newcomers because it is common practice to illuminate tanks with blue light at night.

This allows fish owners and passersby to view the fish without disturbing them. Additionally, blue lights are attractive. They improve the tank’s visual appearance.

However, if you want to know whether or not you can leave the lights on at night, the answers you get will vary. Consider the following:

1. It May Change How Your Fish Behave

Many aquarists are hesitant to leave blue LEDs on at night because artificial lighting is dangerous, especially when it persists.

Grace Fuller (Detroit Zoological Society), Mary Ann Raghanti (Kent State University), and Pam Dennis (The Ohio State University and Cleveland Metroparks Zoo) investigated this topic.

They published an article in Applied Animal Behavior Science that looked at the impact of artificial illumination on nocturnal animals.[1]

They found that animals housed under blue light were less active than those reared under red light. This paints blue LEDs in a negative light.

A story in Science Daily (sourced from Forschungsverbund Berlin) took things a step further and noted that guppies exposed to artificial light at night took more risks during daytime.[2]

This compromised their safety because it exposed the creatures to predators. People gravitate towards blue light because it is dimmer than other colors.

But guppies exposed to low light levels in the study mentioned above manifested the same risky behavior seen in fish that lived under bright lights at night.

Ralf Kurvers (MPI for Human Development) led the team that performed the study. You have every reason to approach this issue with caution.

Admittedly, a paper in Environmental Pollution (Jaroslaw Kobak, Magdalena Czarnecka, Milena Kulasek, Lukasz Jermacz, Paulina Glazinska) emphasized the fact that artificial light at night doesn’t produce physiological changes.[3]

Instead, it creates behavioral modifications. But behavioral changes in an aquarium should worry you because they add an element of unpredictability.

2. It Will Encourage Algae Growth

Even though blue light appears dim, it delivers enough energy to generate a photosynthetic reaction in plants, including LED light.

This is good because plants consume oxygen in the dark, which you don’t want. They do the opposite in lit environments, generating oxygen while taking in carbon dioxide.

Therefore, you can maintain optimal oxygen levels in the water by keeping the blue light on all night.

Unfortunately, conditions that benefit plants will also boost algae growth. This is the primary threat that encourages some aquarists to reject blue lights at night.

However, you can control algae growth by keeping the blue LED dim.

Some blue lights are too bright and intense, which is why the algae population runs amok in the tanks they inhabit.

Many amateur aquarists don’t realize that a blue LED has controls that lower the light’s intensity. You can avoid heavy algae infestations by restricting the blue light’s intensity.

3. It May Disturb Your Fish’s Sleep

Many fish owners believe that fish cannot see blue light, which is why they are comfortable with leaving the blue lights on.

But they couldn’t be further from the truth. Fish can see blue light, just as they can see the reflection of the moon in nature.

And just like humans, fish need darkness to sleep properly. If you leave the blue lights on all night, it will disturb your fish, stressing them in the long run. That is also true for aquarium shrimp.

Obviously, this is also true for normal light, which is why the general recommendation is not to leave the aquarium lights on 24/7.

Bright lights at night may prevent your fish from sleeping properly (even if they are blue in color).

Why Do Some Aquarists Choose To Leave The Blue Light On?

Regardless of the previously mentioned reasons, some fish owners choose to leave their blue lights on all night. These are usually the reasons for this:

1. It Enhances The Fish’s Colors

Despite potential drawbacks, blue light is still appealing to many aquarists because the color relieves stress.

It calms the fish, which makes sense because it essentially mimics the moon. Some people use it to mark the transition between day and night.

The fact that it allows you to view the aquarium’s inhabitants without disturbing the fish is a bonus.

Don’t be surprised if the tank becomes more attractive. The blue light will enhance the vibrancy of the fish’s colors, which is why many use blue light in their betta fish tanks.

Naturally, you should approach blue lights on a case-by-case basis. Don’t hesitate to remove them if you observe a negative response among the fish.

2. People Feel Comfortable With LEDs

If you want to leave the blue light on all night, LEDs are your best option. First of all, the best brands can switch between white and blue light with the press of a button.

You can program some of them to gradually shift from white light during the day to blue light at night.

And if you don’t want to leave the blue light on all night, you can program the LED to shut off a few hours after you sleep.

Secondly, you can go an LED on all night without raising the temperature. This is where incandescent bulbs fail.

You can’t trust them to illuminate the tank without causing the temperatures to spike. LEDs are cool enough to stay on for long periods without affecting the temperature.

3. Some Own Nocturnal Fish

If you have nocturnal fish, you can leave the blue lights on all night. The blue LEDs are bright enough for the fish to hunt and feed without antagonizing the creatures.

Some fish owners keep the blue light on all night because it makes their aquariums look beautiful.

Is It Expensive To Leave The Blue Lights On?

No, it isn’t. The lights are usually one of the most inexpensive components of an aquarium because you don’t use them all the time.

After all, fish typically spend their nights in darkness. At worst, you will spend a dollar or two each month on a conventional tank’s lighting needs.[4]

Keeping the lights on all night won’t raise the cost significantly, especially where LEDs are concerned. LEDs are highly efficient. You won’t notice the impact on your wallet.

When Should I Turn The Blue Light On/Off?

Interestingly, it doesn’t matter. The key to maintaining a healthy aquarium is to give the fish an equal amount of daytime and nighttime.

You don’t have to keep these periods perfectly equal. Aim for 8 to 12 hours each.

You can switch the blue light on in the evening when the sun goes down. You can also use it much earlier in the day when the sun outside is still up.

The goal is to use the blue light to transition from one section of the day to the other. This is why many fish owners turn it on at 6 pm and off at 11 pm.

You are free to partition your tank’s daytime and nighttime in any way you want. But the fish don’t actually need the blue light.

People only use it because they want to see the aquarium at night without disturbing the fish. Therefore, you can switch it off the moment you go to bed.

Better yet, program the blue LED to turn off at a specific time. Create a lighting schedule and stick to it.

Can Fish Sleep With Blue Lights?

It mainly depends on the brightness.

If the blue light is bright enough for you to see your fish’s every move without straining, it is probably too bright, and the fish may not sleep.

Again, you should observe the fish’s behavior. If they can sleep with the blue light on, leave it on. If they can’t sleep, turn the blue light off.

An excellent YouTube video explaining why it can be dangerous to leave the blue light on all night.


In most cases, it’s best not to leave the blue lights on all night. This is especially if the light is strong, as it will disturb your fish’s sleep.

Moreover, this action may encourage algae growth. If you keep forgetting to turn off the lights, you can use an automatic system that will do it for you.

These types of LEDs are quite common these days and they don’t cost much. Your alternative is to use the dimmest level possible.