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Do Fish Need Light at Night or Can you Turn it Off?

Aquarium light is important for plants and to see our fish in the fish tank, but I did not know if it were possible to leave the light on for the whole night. So, do fish need light at night?

Quick Answer

Aquarium fish do not need tank light and it is best that you turn it off during the night. Leaving the light on can cause stress to fish as they need a period of darkness to sleep. Too much light will cause algae to rapidly grow and make your tank look dirty.

So the short answer is no, do not leave your lights on. But what if you would? There are a couple of different crucial factors you should know.

Light During the Night Causes Stress to Aquarium Fish

Not turning off the light during the night for a longer period of time causes stress to all aquarium fish.

In an aquarium we always try to make the circumstances as natural as possible. This is to give our fish the best possible conditions for them to live in. Remember that we are responsible.

A big part of this is mimicking the day and night cycle, and this applies to nocturnal fish too, ones that like their moonlight. A bit of blue light, a night light so to speak, may be fine, but that’s about it. It’s all about maintaining circadian rhythms.

When the fish have no “night” they will get stressed and display symptoms like lack of appetite, constant hiding and fighting each other. When you let this go long enough they might even die from their symptoms.

You will notice these symptoms soon enough, however there will be another problem that you will not be able to ignore much sooner: algae, which you will notice in the daytime.

More on this later.

If you do not already have a timer for your lights, make sure to get one! They are really affordable and will allow you to accurately control your lights without any effort.

I recommend this simple timer that’s available on Amazon. Check it out, it’s definitely worth it!

Other Effects Too Much Aquarium Light Has on Your Tank


Personally, I have accidentally left my aquarium light on for a weekend. When I got back on Sunday evening, there was so much string algae in my aquarium!

This is what happens when there is too much light in a freshwater fish tank, especially if direct sunlight is involved.

Algae will be able to out compete all live plants that you have in your aquarium and affect oxygen levels. Especially when there is a lot of nutrients readily available.

You might think that having (fast growing) live plants in your aquarium will counter this problem. I thought the same but my aquarium was proof that the algae still thrive when there is too much light.

Therefore, whenever you see algae, make sure to cut back on the amount of light. This is an effective way to battle algae in your aquarium. Here are some great algae-eating fish!

Looking for more information on aquarium fish?

In that case, make sure to check out our aquatic fish page. There you’ll find everything you need to know about aquarium fish, shrimp & snails.

Are You Running CO2 injection?

I am running CO2 injection on one of my tanks, which only adds pressurized CO2 when the lights are on, something that helps with plant growth in my aquarium setup, combined with a good fish tank light.

This means that my CO2 will run out way quicker when my lights are on 24 hours per day instead of the 10 hours they are running right now.

Also, live plants need a period of darkness as well.

Therefore it is important to turn your aquarium light off during the night. During this period the photosynthesis stops and contribute to the day and night cycle all plants have.

How Many Hours Can you Leave Your Light on Per Day?

When your aquarium is still new, I always recommend starting at around 8 to 10 hours of light per day. This is an average amount of light, and you will be able to see what effect it has on your aquarium.

If nothing is happening after a couple of weeks (nothing algae-wise) you can increase the duration of light.

Do this gradually, with increments of 1 hour. You can do this for a while, but I would stop when you are at 12 hours of light per day. This is a lot of light for just about any aquarium, and remember, you don’t want to disturb your fish’s sleeping schedule or behavior.

When you start and you notice a lot of algae in the aquarium, you probably have too much lights. This can mean that you leave the light on for too long, but it can also be that your light intensity is too bright.

Figure out what the problem is and fix it. Knowing how much light your aquarium plants need is very important.

What if you do not have live plants in your aquarium? Well than the light can only cause algae. That brings us to the next question, how important is the light for your fish?

What do Fish Get from Aquarium Light?

Your fish do not need the light. They can easily navigate and find their way without it, but light and light color may still affect fish health.

The light is just for us to watch our fish, grow our live plants and make sure the tank is doing well and staying healthy.

This means that theoretically you could have an aquarium without light. But what would be the point right?

You want to be able to see your fish.

What you can do however, and this is something that some people in the hobby do, is only turn on the light whenever you are near the aquarium.

This way, you are saving electricity (even though LED lights do not use that much power anyways). I have read that people only turn on their aquarium light in the morning before work, and as soon as they get home.

This is possible as long as you do not forget to turn off the light, and the total amount of hours is not extremely high.

Light Consistency is Important

When we are trying to keep all the conditions as natural as possible for our fish, it is important that the aquarium light turns off and back on again on a predicable schedule, to help mimic daylight and a natural circadian rhythm.

This is best achieved using a timer.

I really do not understand how there still are people that are not using a timer for their aquarium lights.

It just allows you to make sure your fish and plants get the same amount of light every single day without you having to worry about it.

They are also not expensive at all, I always recommend this programmable timer on Amazon. As you can see it’s very affordable.

Because you have your light on a timer, you will never forget to turn off the light at night. This way, it will not happen that you are beaming intense light at your fish all night long.

Furthermore, you will notice that your fish get used to the schedule as well, especially when it comes to the the low light conditions or complete darkness associated with nighttime.

I notice this especially with my rummy nose tetras. When I turn on the light in the middle of the night (yes I have done that do not judge please) their noses are not colored at all.

When I watch them as soon as the light comes on on their natural schedule their noses are way more colored.

This is the same for feeding on a schedule. The fish will expect to get the food at a certain period of the day.

Tips for Choosing an Aquarium Light

  • Knowing the differences between different types of lights, such as metal halide and LED, can help you make a more informed choice.
  • If you are looking for the best possible results, both for plant growth and fish coloration, using full-spectrum lights is recommended.
  • Always consider how much light energy your plants need compared to the amount of energy that the light in question puts out.
  • The size of the tank matters as well. The deeper the tank, the more powerful the light needs to be to reach the depths.
  • Growing low-light aquarium plants always makes things easier too!

My favorite fishtank products that make life easier

I am so happy you enjoy this post so far! You will also definitely like my product recommendations that will make your fishkeeping experience so much better. I’m 100% sure you’ll love them!

  • Without a gravel vacuum, like one from Aqueon, cleaning the substrate of your tanks is near impossible. Whenever I want to remove some of the sunken detritus from the bottom of my acrylic tanks I’m happy I’ve got one of these.
  • It’s no secret that I do not like nutritious aqua-soil. It makes a mess and only works for a given amount of time. Instead, I always use a liquid aquarium plant fertilizer. Everyone who keeps live plants needs it, it’s not that expensive and makes your plants grow better.
  • I love keeping plants, but planting and reorganizing my aquarium was difficult until I got a set of these tools. It’s much easier to plant any kind of plant compared to using my thick fingers.
  • Ever since I’m able to accurately test my water parameters, including the pH level, keeping fish became less stressful. Before I was always stressed that my water parameters were wrong, but with a kit such as the API Master kit, I can measure this. It really is essential to successful fishkeeping.
  • The more you know about your aquarium, the better! Temperature is crucial for the health of your fish. A thermometer will also show you whether your heaters are still working correctly. It will give you more insight and more peace of mind. It’s an easy way to ensure that you’re providing your fish with the tropical temperatures they need.

Feature image – Author: Emilia Murray Licensed under: Some rights reserved No changes made
Aquarium image – Author: Pete Brown Licensed by CC2.0 Cropped