What Does Blue Light In Fish Tank Do? Can Fish See Blue Light And Is It Harmful At Night?

Does your aquarium have a blue light that you’re not sure what to do with? It creates a nice ambiance, but when should you use it? Should you only use it at night, or can you keep it on all day?

To know the answer to these questions, you’ll need to know what blue light is. So, what does blue light in fish tank do?

Keep reading to find out.

It’s very common for aquarium lights to come equipped with both a daylight bulb and a blue bulb. As you might have guessed, the blue light simulates nighttime.

If you don’t receive natural moonlight in your home, blue light is a good alternative.

Blue light helps your fish to calm down and rest. Your fish will be less alert than they are during the day because they feel safer from predators.

Likewise, your nocturnal fish will feel more comfortable coming out of hiding. They feel safe under the cover of darkness and will feel freer to swim about.

Nocturnal fish prefer a light blue light over complete darkness. So, it’s better to use a blue light than to simply turn off your daylight bulb. Blue light gives your nocturnal fish enough light to see while maintaining safety. It also encourages the physical activity of your nocturnal fish, so you’re more likely to see them out and about.

Is Blue Light Good For Fish At Night?

It’s important for fish to have a natural day and night cycle like us. If that cycle becomes disrupted, it can lead to confusion and illness. Luckily, it’s easy to emulate the day and night cycle. Use a regular daylight bulb during the day, and a blue bulb at night.

It’s not good to keep the regular daylight bulb on 24/7. Fish need to be aware of the passing days or they can become sick and stressed.

Blue lights are also nice because they offer a gentle transition from day to night. Many people don’t use blue lights, they simply turn the daytime lights off at night.

This is okay, but it can be stressful for the fish. Going from bright daytime light to complete darkness can be scary and startling for fish. If you switch the daytime light to blue light, the transition is gentler.

Can I Leave Blue Light On In A Fish Tank?

You shouldn’t use blue light shouldn’t all day. It doesn’t have all the wavelengths needed to keep your fish healthy. It may also confuse them because it doesn’t look like natural sunlight.

During the daytime, light up your tank with daylight bulbs or natural sunlight.

Is Blue Light Harmful To Fish?

The only time that blue light is harmful to common aquarium fish is if you use too much of it. You shouldn’t expose your fish to daylight 24/7, and you shouldn’t expose them to blue light 24/7.

As we mentioned above, blue light doesn’t contain all the wavelengths that fish need to stay healthy. So, if you only provide them with blue light, they are likely to become sick.

Yet, blue light can be stressful and disruptive for larger predatory fish. If this is the kind of fish you house in your aquarium, you should avoid using blue lights. Blue lights aren’t natural in the wild, so they can disrupt the feeding patterns of predatory species.

Blue Lights Are Good For Plants

If you plan to keep a planted tank, you should know what kind of light they need. Many new aquarists don’t think about the quality of light they’re providing their plants. Some plants are hardy and will thrive under any kind of light. Others need more care and are picky about the light they receive.

Blue light easily absorbs into the leaves and stems of your plants. With blue light, plants experience better development and will grow strong and healthy. Best yet, your plant’s colors will pop more.

Blue Lights Don’t Encourage Algae Growth

Algae growth occurs for many reasons, but blue light is not one of them.

Daytime light can contribute to algae growth through photosynthesis and heat. Algae needs a light source to generate energy.

Algae photosynthesizes by absorbing the red-yellow spectrum of light. White light is what generates this spectrum.

Technically, algae can grow with blue light as well, but its growth drastically slows down. So, we don’t generally associate blue light with algae growth.

White light also encourages algae growth by heating up the water. Algae grows better in warmer waters, so it’s more likely to grow during the day when it’s exposed to white light. Blue light is cool and doesn’t generate much heat.

Why Else Is It Nice To Have A Blue Light?

Besides being good for your fish, it’s nice to have a blue-light-equipped aquarium. The light provides a beautifully calming aesthetic at a time when the rest of your house is dark.

Imagine walking through your dark house at night. Now imagine seeing your favorite fish flitting about under blue light. The sight will calm you just as it calms the fish.

Plus, you’ll see some of your fish’s colors better under blue light. White light accentuates certain colors, while blue accentuates others. You might be missing out by not using your blue lights.

For example, white and black fish stand out far better under blue light than they do under white light. Under blue light, they almost seem to glow. Red, green, and blue colors also seem to pop more under blue light.

Blue lights might save you money as well. Perhaps you didn’t know that fish should experience the day-night cycle. If you kept your daylight bulbs on 24/7, it was probably racking up your electricity bill. Blue lights don’t put out as much heat as daylight bulbs, so they spend less energy.

How Do You Know How Much Light To Provide?

We keep emphasizing the importance of the day and night cycle for your fish. But what exactly does this mean? How much light should you be providing your fish?

Luckily, the answer is simple!

Fish follow the same day and night cycle that we do as humans. Their sleeping schedules likely differ from other fish in the tank, but this doesn’t matter.

Use your lights in a way that mimics the natural rise and fall of the sun. If you’re able, turn the light on at sunrise. If you don’t get up that early, then turn the lights on as soon as you wake up. Likewise, you should switch to blue lights once the sun begins to set.

Maybe you have a weird sleeping schedule yourself. Or maybe you’re afraid you’ll forget to change the lights. You can always buy a timer that connects to your aquarium lights. You can choose which time you want each light to come on and off throughout the day.


Blue light simulates nighttime in your fish tank. It gives diurnal fish a chance to rest and tells nocturnal fish when to come out to feed.

Using blue light at night is perfectly safe. It provides your fish with a nice nighttime environment. It also allows you to still see your fish after dark.

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