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What Does Blue Light In Fish Tanks Do? (6 Essential Benefits)

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Almost everyone has seen blue lights in an aquarium at least once, whether it’s at a restaurant or at a friend’s house. But not everyone knows what it does.

Since I have been asked this question countless times in the past, I decided to devote an entire article to this topic.

In this article, I will explain the purpose of blue lights in fish tanks, what they actually do, and how to use them correctly.

Let’s dive right into it.

What Does Blue Light In Fish Tanks Do?

Blue light in fish tanks helps fish transition between day and night, benefits nocturnal fish, improves visibility, and reduces stress. However, leaving blue lights on all night is not recommended as fish need darkness to rest.

Many home and commercial aquariums use blue light these days. The color has various positive and negative consequences.

As for now, aquarists have made the following observations:

1. Blue Light Mimics The Moonlight

Blue LEDs mimic moonlight. In fact, some manufacturers call their blue lights ‘Moonlight LEDs’ to emphasize the specific role they play.

The comparison makes sense because the wild exposes fish to moonlight every day.

On the one hand, this isn’t always a good thing because moonlight increases a fish’s predation risk.

Some species become less active on bright moonlit nights because they don’t want to fall prey to larger animals.

You also have predators that starve because the increased illumination has sent their prey into hiding.[1]

On the other hand, some studies have found that moonlight can improve larval growth in fish.[2]

Variables like the level of illumination and increase or decrease in predation will come into play.

Predators are not really a challenge in aquariums because aquarists endeavor to keep vulnerable species away from the creatures that typically hunt them.

Nonetheless, you can recreate the behaviors and mannerisms seen in the wild by adding blue lights to your aquarium.

2. Blue Lights Help Transition From Day To Night

Fish in the wild will gradually transition from day to night time. This is also true for humans.

The sun doesn’t switch off suddenly, plunging the world into darkness. The sunlight fades gradually until nighttime takes over.

You should endeavor to replicate this process in the tank. This is why many aquarists turn the blue light on at 6 ppm and off at 11 pm.

This gives the fish a moment to adapt to the darker environment. The same thing happens at dawn. 

It can take a salmon thirty minutes to grow accustomed to bright light (one hour for low light).[3] It won’t benefit their health to shock these creatures by suddenly turning the lights on or off.

Blue lights help with this transition. You can keep them on for a few hours at night before you go to sleep.

They will lull the fish into a relaxed state, allowing the creatures to rest once you turn the lights off. That is also true for other aquatic creatures, including shrimp.

Some people don’t have the patience to switch between conventional white lights and their blue counterparts at dusk and dawn.

But you can program your lighting system to perform this transition automatically without your direct involvement.

You can turn on the blue light for an hour or two before bed to help your fish get used to the night gradually.

Pro tip: Leaving the blue lights on all night is actually a bad idea, as it prevents your fish from sleeping properly.

3. Blue Light Benefits Nocturnal Fish

Fish don’t need light at night. Professional aquarists will encourage you to maintain 8 to 12 hours of darkness. This allows the fish to rest.

But what about nocturnal fish? Nocturnal species, such as cardinalfish, are only active at night.

They can also do without artificial lighting. After all, even in the wild, you have days when the moon doesn’t emerge. 

And yet, they can still hunt. Fish have a sensory system called a lateral line that detects motion in water.[4]

This system allows them to catch prey and escape predators in the dark. 

If you must add a light, many experts recommend red because it is bright enough to illuminate the fish’s surroundings but not so bright that it overwhelms the creatures.

You also have those fish that only emerge at night because they have timid personalities.

A conventional white aquarium light would send them into hiding, even at night. Red LEDs are dull, warm, and inviting. They can coerce nocturnal creatures out of hiding. 

Blue is equally appealing. Actinic blue is dim, yet it can penetrate the water more efficiently than its white and red counterparts.

This color will enable nocturnal fish to hunt without distressing them. Some people keep their blue LEDs on all night, but that practice is discouraged. 

The diurnal species in the aquarium will suffer because they need several hours of darkness to rest.

However, some aquarists have fish that don’t react to blue or red light at night. They sleep peacefully.

4. It increases The Aquarium’s Visibility At Night

Fish need the darkness that nighttime brings. It allows them to sleep. However, some fish owners don’t rear fish out of the goodness of their hearts.

Some rear fish because they enjoy watching the creatures as they hunt, play, and breed in their natural habitat.

This is a problem if you have a home aquarium and yet you spend the day at an office away from home.

You can only see your fish at night once the sun sets. But doesn’t that mean exposing them to five or six additional hours of white aquarium lighting?

Not necessarily. One of the reasons you’re so familiar with blue light these days is that many restaurants use it.

The color allows people to view the activities in the aquarium without disturbing the tank’s inhabitants.

Many businesses use blue aquarium lights to attract visitors.

5. Blue Light Reduces Stress Among Fish

Blue lights fight stress in fish. It sounds ridiculous, but it’s true. 

G.L. Volpato and R.E. Barreto published a paper (Brazillian Journal of Medical and Biological Research) showing that blue light could prevent stress in Nile Tilapia.[5]

They authenticated the theory by subjecting Nile Tilapia to confinement stress. The plasma cortisol levels in Nile Tilapia exposed to a blue light environment did not increase.

In other words, this isn’t a tenuous theory originating from sketchy aquarists with questionable anecdotes.

Scientific research shows that blue light makes fish calm.

It soothes the creatures, which is why aquarists use the color to create a transition between day and night. 

Blue LEDs create a conducive environment for fish to sleep peacefully once you turn off all the lights.

This is one of the reasons why aquarists are so enthusiastic about blue lights, even those who only keep a single betta fish in their tank.

It is better to choose LEDs because they have the same effect as bulbs but they do not change the water temperature.

6. Aquatic Plants Can Use Blue Light For Photosynthesis

As I previously discussed, blue light is good for plants. The color drives photosynthesis in vegetation.

Plants will only consume CO2 and produce oxygen when they have light. In the dark, they do the opposite.

In other words, dense foliage can make an oxygen deficiency worse if you have prolonged seasons of darkness.

A blue light encourages photosynthesis to occur at night without disturbing the fish.

It is also worth noting that blue suppresses extension growth, producing shorter plants with smaller, thicker leaves.

Naturally, anything that boosts plant growth can also elevate algae growth. Some people believe algae cannot grow under blue light, but they are wrong.

Algae may grow slower in aquariums with blue LEDs, but it will still grow, especially if the blue LEDs are intense and persist for several hours.[6]

What Kind Of Blue Light Should I Pick For My Aquarium?

If you have decided to equip your tank with blue lights, I highly recommend sticking with LED lights. These will prevent your aquarium from overheating.

Another important factor is a timer. Without one, you’ll keep forgetting to turn off the lights. This has happened to me countless times before.

This is why I ended up going with the Hygger Full Spectrum Aquarium Light (link to Amazon). It is very affordable and gets the job done.

And if you’re still hesitating, here’s an excellent review of this product by ‘Prime Time Aquatics’:


Blue lights actually provide aquatic creatures with an environment somewhat similar to what they experience in the wild.

This is done by imitating the hues of the moon. Many aquarists use this type of light to help their fish gradually transition from day to night.

However, some fish owners use these lights to view their aquarium late in the day, after sunset. This is the main reason why many restaurants use blue LEDs.

Yet, leaving the lights on all night can actually do more harm than good, as it prevents the fish from sleeping. Just like humans, these creatures need a few hours of darkness.