Skip to Content

Do Aquarium Fish Need Light at Night? The Truth Revealed

Do Aquarium Fish Need Light at Night? The Truth Revealed

Aquarium light requirements are among the things that confuse novice fishkeepers the most. For example, many beginners wonder do aquarium fish need light at night, or should they spend the night time in the dark. 

Most living beings need light to survive. Without the light, there would be no life as we know it. Even most microorganisms need some amount of light to develop and multiply. 

However, there are some limits, and too much light or light at inappropriate times can be harmful. 

So, do aquarium fish need light at night? Let’s explore this topic and get all the answers! 

Do Aquarium Fish Need Light at Night?

Nature has a day and night cycle (in most areas of the Earth, at least). Most living beings have a rhythm that’s based on that cycle. 

This means that no, fish do not need light at night. Quite the opposite – they need darkness to sleep and develop properly.

While many people think that complete darkness would confuse their fish, the truth is that leaving the light on during the night could lead to more stress or even some health concerns!

Of course, there are exceptions to every rule and the same can be said for this topic, as well. This is why we need to get deeper to fully understand do aquarium fish need light at night, why not, and why this is so.

I’ll start by answering one of the most common questions. 

Why Are My Fish More Active when the Light Is Off?

guppy fish

Even if you’ve just started your first tank, you’ve probably noticed that most fish are more active during the night than during the day. Or rather, they seem to be more lively when the lights are off.

There is a biological reason behind this. 

Many fish that hunt their prey or scavenge for food do this during the nighttime. Smaller fish feel safer when there are no lights, and large predators can remain unnoticed by their prey.

 Of course, not all fish are nocturnal and there are diurnal fish as well. Diurnal fish prefer to stay hidden during the night and sleep in caves or in other hiding spots.

Whichever of the two types of fish you have, it’s easy to see that having a period of darkness is important for them to sustain their natural activities.

Effects of Too Much Light on Your Tank

Whether you decide to leave your tank light on or if you consider providing your fish with direct sunlight, too much light can have some adverse effects on your aquarium inhabitants.

In fact, most tanks will thrive if you provide them with 8 to 10 hours of light during the day, not more. If you’re new at the aquarist hobby, this is the good amount to start. Of course, monitor the condition of your tank so you can see how this affects your fish.

If you have live plants, the amount of light might be a bit longer. However, I would still start with 10 hours, then slowly increase this amount to make sure there are no algae. 

If you immediately start giving your fish light during the night, below are some of the side effects you can expect: 

1. Algae

All tanks will have algae to an extent. This isn’t even a bad thing. Algae are a food source for many fish and they can help keep the water oxygenated. 

However, too much light can cause algae overgrowth. Too much of anything isn’t a good thing, and the same goes for algae. 

When your tank has enough algae for you to notice green buildup on the top of tank decorations or even on top of the tank, you know your aquarium has received too much light.

In fact, algae overgrowth is the main sign that you have gone overboard with providing your fish tank with light

Having too many algae can result in a lack of oxygen and disrupt water parameters. This can severely harm the quality of life of your fish.

Algae can also grow if your light is too strong or provides too much energy. Sunlight is a good example of this. 

2. Too High Temperatures

Most types of light will cause heat to a certain extent. This is especially the case with incandescent lights which are to be avoided for this exact reason.

However, any type of light will cause heat to an extent, especially if you keep it on throughout the night. 

Even tropical fish and shrimp, such as cherry shrimp, that thrive in warm water can die if the water gets too hot. Things get even worse if you have coldwater species, such as barbs and tetras!

While this isn’t much of a concern if you have LED and fluorescent lights, I still wouldn’t risk leaving the light on for too long, especially if there is a heatwave.  

3. Improper Sleep Cycle

Fish don’t sleep in the same way as humans – but they still have a sleep cycle. In nature, darkness is necessary for fish to sleep properly.

Even fish who sleep during the day still need a period of darkness in order to know when is the time to get some rest

Without darkness, fish can become stressed out. This can lead to a decline in their health, and if this keeps on happening, they might even die!

When you catch yourself thinking, ‘Do aquarium fish need light at night?’, just think about humans who live near poles where there are midnight suns! They often complain about not being able to sleep, and both their mental and physical health can be ruined. 

The same happens to fish if you keep the light on during the day.

4. It Harms the Plants

While aquatic plants need light to photosynthesize, they also need a period of darkness for the process to end properly. 

Plants respire during the dark, when they’re not photosynthesizing. Just like photosynthesis, this process is necessary for their wellbeing.

Not just that, but too much light can create free radicals inside the plants. Free radicals are harmful not just to humans and animals, but to any living being, including plants.

Overtime, without a proper darkness period, your plants might die.

If you’re only keeping plants inside your tank, or if the light isn’t too bright, you can keep the light on for 12 hours instead of 10. Just don’t keep it on for much longer.

If you wonder what this has to do with fish – if all the plants die, your fish will likely die, as well, especially if you’re not taking care of oxygen inside the tank. 

5. It Takes Away CO2

If you’re running CO2 injections, this can be another reason why you must give your tank a proper nighttime.

CO2 injections add pressurized CO2 for as long as the light is on. If you keep the light for too long, you might run out of CO2. 

In order for your injections to last longer and for your plants to thrive on CO2, you need to provide a proper period of darkness for your tank.

6. It’s Unlike Natural Habitat

Even captive-bred animals still have their natural instincts. Their biological clock is designed to follow a cycle of light and darkness. It is written in their genes. 

When you keep the light on during the day, you are messing with the fish’s natural environment. It is natural to have a period of darkness, and you have to provide this to your pets.

Animals that are kept inside your home don’t have natural conditions to live in. It is up to you to mimic their natural habitat so they can live the happiest life possible.

When Do Fish Need Light at Night?

red wag swordtail with mollies

While you should keep the light off during the night, there are certain occasions when keeping the light on for a longer time is a good thing.

It’s important to note that you should never keep the light on 24/7, unless we’re talking about a specialized night light that acts like moonlight. 

However, at a certain point, your aquarium might benefit from shorter periods of dark. This might mean you’ll keep the light on during some periods of the night (but not all of it).

Below are some common situations that might require more light than usual: 

1. During Breeding

When it’s time for them to breed, fish might need more light than normal, but not for reasons you might expect. 

Many fish species lay eggs during the night or in the evening. This is risky, as carnivorous fish can come and eat the eggs when the dark falls. By keeping the night light on, you’ll deter the predators and increase the chances of fish hatching.

Night light might also help coral spawn, as their reproductive cycle is often connected with moonlight. 

2. When Fish Are Sick

If your fish is sick from fish itch or some other common condition, a dim light can help them recover more quickly. 

Light can boost fish’s immune systems and give them comfort

Also, light can help you keep a close eye on your fish so you can have a better idea about their health status.

3. If You Have Nocturnal Fish and Shrimp

Nocturnal fish and shrimp, such as plecos or amano shrimp, don’t sleep during the night, or at least not during its entirety. In order for them to be active and navigate the tank, they need at least a bit of light.

In nature, these fish would orient themselves with the help of moonlight. You need to replicate this by providing them with a proper nightlight.

Even if you have some diurnal fish together with nocturnal ones, you should still provide them with night light. Most diurnal fish won’t mind nightlight, as they sleep in the moonlight in nature, as well.

Also, nocturnal fish tend to feed during the night. Moonlight helps them locate food and keeps them on schedule. 

Nocturnal fish might also become aggressive toward sleeping fish. When there’s moonlight, they won’t be as aggressive. 

Finally, by keeping the night light on, you can monitor your nocturnal fish and ensure they are behaving properly.

The Best Type of Night Light

saltwater fish tank

There are many types of night lights you can get for your fish. However, from my experience, the best type of night light is blue light.

Blue light mimics the natural moonlight. It can help them feel comfortable as it’s the closest thing to their natural environment they can get. This might even help them grow faster and it can help their mating and feeding habits.

Also, this type of light is not too bright to mess with the fish’s sleeping schedule, but it’s bright enough to help nocturnal fish hunt for food.

Can I Leave My Aquarium Light on 24/7?

While I have given you the answer to ‘Do aquarium fish need light at night’, maybe I haven’t been clear enough about leaving the light on all day and night long. 

It is never a good idea to keep your light on 24/7, unless you plan on having a blue light for the nighttime.

There is not a single occasion in which it would be a good idea to keep the light on throughout the night for prolonged periods of time. 

Even nocturnal fish will hate having the bright light on, as they’ll feel confused and stressed out due to the lack of darkness. 

When you consider all the other downsides of keeping the light on for too long, it’s better not to risk it. Not to mention that your utility bill will be grateful for turning the light on for 12 hours a day!

Final Words

If you think wondering ‘Do aquarium fish need light at night’ is funny, don’t. Many experts are unsure of the exact reasons why too much light is harmful, and it’s easy to see how one would think nocturnal fish need bright light to see. 

However, too much light is harmful for all tank inhabitants, from plants to snails and fish. 

When wondering about such things, always think about fish in their natural habitat. As you should always strive to give them the most natural conditions possible, this also means giving them a proper nighttime.  

To make things as natural as possible, I would also invest in a good blue light. This will help make your fish thrive!