Do Fish Need Light At Night? [Why Not + When It’s Needed?]

Image of fish in light at night

Fish don’t need light at night as they need darkness to rest. Light at night disrupts their sleep, leading to stress. Excessive lighting also causes algae bloom, overheat the water, and affect the plants. Light is only needed at night during spawning, sickness, or while feeding nocturnal fish.

Adequate lighting is crucial for maintaining a healthy ecosystem in aquariums. However, keeping the lights on at night can prove disastrous to fish. So, let’s understand more about this.

Why Shouldn’t You Keep Aquarium Lights On At Night?

Lighting is essential if you have a planted tank. Aquarium plants need light to flourish.

However, keeping the lights on at night cause more harm than good.

Here are a few reasons why you should avoid keeping aquarium lights on at night.

1. Affects sleep.

Fish follow the same circadian rhythm of day and night as we humans do.

Hence, they need darkness at night to rest and sleep properly.

Moreover, there’s no light in the fish’s natural habitat.

So keeping the lights on during the night in an aquarium disturbs their circadian rhythm.

Fish won’t be able to sleep peacefully if there’s light at night. Lack of sleep stresses the fish.

Stress leads to aggressive behavior, fatigue, appetite loss, crankiness, and weakening of the immune system.

Excessive stress for a prolonged duration also leads to illness and can even be fatal to the fish in extreme cases.

2. Doesn’t mimic fish’s natural habitat.

Mimicking the natural habitat of fish helps to keep them healthy.

Fish experience day and night cycles in their natural environment.

So it’s good to keep the aquarium lights on during the day and switch them off at night.

This mimics the natural habitat of fish and makes them feel safe and comfortable.

3. Causes algae overgrowth.

Another adverse effect of keeping lights on at night is that it causes algae proliferation in the aquarium.

Algae need light to thrive, coupled with nutrients.

If the aquarium light is kept on at night, algae will grow and multiply faster. In no time, algae will cover your entire tank.

Moreover, if there are excessive algae in the aquarium, they start competing with the plants for the resources such as carbon dioxide.

Since algae are more aggressive, they effortlessly dominate the plants. So plants can perish due to nutrient deficiency.

Besides, an algae bloom makes the tank look dirty.

So it’s better to switch off the light at night to control the algae overgrowth.

4. Affects plant growth.

Plants need adequate lighting to carry out the photosynthesis process.

Most of the aquarium plants need approximately 12 hours of lighting to make their food and thrive.

Overexposure to light harms the plants as they also need darkness to respire.

Keeping the aquarium lights off at night stops the photosynthesis process. It creates a day and night cycle that plants need.

Besides, excessive lighting creates free radicals in plants.

This generates reactive oxygen species that may prove harmful to the plants and destroy them.

Problems can also arise if you are running a CO2 injection in your tank.

If the lights are kept on all day long, it adds pressure on the CO2.

This results in the CO2 running out way quicker than when the lights are switched off at night.

6. Overheats the aquarium water.

Keeping the light on during the night overheats the aquarium water.

Aquarium lights are usually kept on during the day for around 8 to 10 hours daily.

So keeping them on at night leads to excess heat generation.

Incandescent, VH fluorescent, and metal halide lights are known to generate heat.

In larger aquariums, it’s not much of a concern due to the high water volume.

However, if the tank is small, excessive light heats the water rapidly.

When Do Fish Need Light At Night?

Fish usually don’t need any light at night. It’s because they need a period of darkness to rest.

However, in certain situations, it’s better to have lights at night.

Following are the instances when it’s advisable to keep the aquarium lights on at night.

1. During breeding and spawning.

One of the most prominent reasons for keeping lights on at night is to observe the behavior of fish during the night.

For example, certain tetras and catfish species usually lay eggs in the evening or at night.

Having light at night keeps the eggs safe from predators or parents who may try to eat them.

Even many corals and reef fish, such as damselfish, spawn in tune with the moon cycles.

They spawn when the moon is full or close to full. On the other hand, guppies reproduce their babies in the predawn hours.

In such instances, having moonlight is beneficial as it helps to synchronize their biological rhythms.

2. Aquarium has nocturnal fish.

Nocturnal fish are active during the night, unlike the diurnal fish that sleep at night.

So, if your tank consists of only nocturnal fish or a combination of nocturnal and diurnal fish, it’s advisable to have some light in the tank at night.

Since the nocturnal fish are active at night, they tend to swim in the tank.

So if there’s complete darkness, the chances are that they may get injured.

In addition, navigation is difficult in the absence of light.

Another reason to have a night light is for feeding purposes.

Many nocturnal fish are shy during the day and compete less for food with the day dwellers.

So, having a night light helps you feed the nocturnal fish at night on their regular schedule.

Besides, many nocturnal fish show their aggressive behavior toward the sleeping diurnal fish at night.

Some of the mid-sized loaches are mean toward the peaceful diurnal fish.

Even large plecos sometimes bite the delicious slime coating of fish such as goldfish and angelfish.

Having a night light curbs their aggressive behavior.

Additionally, it will help you keep a close eye on your nocturnal fish to see if they are causing any problems.

3. When fish are sick.

It’s better to have some light at night when one of your fish is sick.

If you have a dim light on during the night, you can closely watch the behavior of the sick fish.

Moreover, lights give comfort to sick fish.

So if you notice your fish in discomfort, you can immediately take some action before it’s too late.

Do Fish Need Blue Light At Night?

Blue lights are suitable for fish as they are not too bright to disrupt their sleep at night.

Moreover, blue light mimics the moonlight. So, it can be used to create a transition from dusk to dawn and vice-versa.

Creating lighting that resembles the fish’s natural environment also helps to regulate their eating and mating habits.

For example, fish tend to grow faster when exposed to sunlight during the day and then dim-lit tank areas at night.

Also, blue lighting is useful when your tank is in a room that doesn’t have access to natural light at night.

Blue light gives an ambient effect to the tank and doesn’t affect the fish and plants.

Besides, blue light is helpful if you have nocturnal fish in your aquarium.

Blue light enables the nocturnal fish to see and forage for food at night.

Also, since they are awake at night, the nocturnal fish easily carry out their daily activities if there’s adequate light.

Related Questions

Do betta fish need light at night?

Betta fish don’t need light at night. They need darkness to rest and sleep peacefully.

Lack of sleep is harmful to their overall growth.

Do koi fish need light at night?

Koi fish don’t need light at night. They need light as well as darkness to grow healthy and strong, just like their natural environment.

Inadequate rest weakens their immune system resulting in health-related issues.

Do molly fish need light at night?

Molly fish don’t need light at night. Instead, they need a dark environment to grow and thrive.

Leaving the light on during the night stresses the mollies. If the tank is too bright, it will affect the health of your mollies.

Do saltwater fish need light at night?

Saltwater fish don’t need light at night.

Saltwater fish need light during the daytime followed by darkness at night to rest and grow.

It’s best to mimic the conditions that your saltwater fish experience in their natural habitat.

Do goldfish need light at night?

Goldfish don’t need light at night. Instead, they need darkness to rest.

Goldfish need around 12 hours of light to maintain their glowing color, followed by an equal number of hours of darkness to sleep and conserve their energy.

Do guppy fish need light at night?

Just like any other aquarium fish, guppies don’t need light at night. They need darkness to sleep correctly.

Keeping the lights on at night disrupts their sleep, resulting in stress, sleep disorder, loss of appetite, and sickness.

Do pond fish need light at night?

Pond fish don’t need light at night. Instead, they need darkness at night to rest and stay healthy.

This is because pond fish have natural cycles like most other animals on earth.

They follow a regular pattern of light during the day followed by darkness at night to rest.


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