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Do Aquarium Fish Need Light or Can They Thrive Without It

I’ve had a 25 gallon (100 L) tropical aquarium for some years now and just last week I noticed that my aquarium light broke. I was unsure whether fish needed the light, so I did my research and this is what I found.

Do aquarium fish need light?

Yes, aquarium fish need light to determine whether it’s day or night. However, the natural light in your room is sufficient. Fish do not necessarily need an extra aquarium light. An aquarium light is to see your fish, and for live aquatic plant photosynthesis.

Luckily fish don’t need an aquarium light if the room it’s in has windows. If the room is completely dark your fish will not do so well in the long term. In the rest of the article I’ll briefly cover why fish can’t live in the dark, whether aquarium fish need light at night and how long you should keep your aquarium light on.

Why fish do need light

It’s best to not keep your aquarium fish in the dark all the time. They need cycles of light to determine whether it’s day and night. This also means that you can’t keep your aquarium fish in the light all day.

If you’ve got a basement or a room without windows but you do not want to buy or install a dedicated aquarium light, a simple solution is to put the lights in that room on a timer. This way, you can still make sure your fish get day and night cycles.

Keeping fish in the dark will lower their immune system, making them prone to diseases and generally weakening them. With insufficient light, fish like goldfish can loose their colors.

A website called pets.thenest also name other symptoms like fish becoming lethargic or not wanting to eat [src]. These are all plausible when fish are kept in either 24/7 darkness or with the lights on.

Do aquarium fish need light at night?

Fish do not need light at night. A period of darkness is necessary for the fish to sleep. Also, leaving the lights on all night could cause algae to rapidly grow and take over your aquarium. That for sure is an unwanted and ugly looking result.

Then there is an option to provide the fish with a night light. Something the fish can use to orient themselves around the tank. With most of us fish keepers it will never be pitch black at night in the room the aquarium is standing.

If your aquarium is in a basement or another room which turns absolutely pitch black adding a small light somewhere in the room to mimic the moon is something that I’ve seen fish keepers do. I would actually also do this when my room would turn pitch black at night. Just make sure the light is not too bright because you don’t want algae growing with the light of your night light.

I’ve actually written an article about this before, which you can check out here if you want.

How long your aquarium light should be on

Fish need day and night cycles with about 12 hours of light and 12 hours of darkness. You could say that this means that you should leave your aquarium light on for 12 hours, but I would not do this. I’ve tried this and asked many people about it, and the conclusion is the same throughout: algae will bloom.

Instead, I would recommend keeping your dedicated aquarium light on for about 8 hours and see how this goes. If there is no algae or even brown algae, raise the light to 9 and eventually to 10 hours. Find what works for you.

Next, the aquarium light is for plants. Plants use the light to convert it to energy using photosynthesis, just like we learned at school. If you’ve got live plants you probably also have an aquarium light. If not, my guess would be that you have plants like Anubias, Java Fern and Java Moss. All plants that can do relatively well with just the light in the room.

Finally, having an aquarium light that turns on and off daily is primarily for ourselves. Without the aquarium light it’s rather hard to see your fish, so that’s why we stick a light on top. With a light, it’s much easier to see your fish, check your tank and enjoy your aquarium in general.

Do aquarium fish like colored light?

I’m unsure whether fish like the colored lights. I do know that fish look very different under a different color light and I’ve had a purple light together with a white light in the same aquarium. Having a colored light is not beneficial for their health. Your light should be white and as close to daylight (5000 – 6500 K).

Some people have asked me whether fish respond to color changing light like those RGB lights. This is definitely something you should not do. Changing lights has the same result as changing any parameters in your aquarium such as temperature or pH: it causes stress.

You want to keep everything as stable as possible so don’t put your aquarium under color changing RGB lights.

Do fish need sunlight?

While fish do need light, which can be sunlight, fish in your aquarium do not need direct sunlight. Placing your aquarium in direct sunlight is something you want to avoid for good reasons. Direct sunlight is so strong and contains a lot of energy, which will be harnessed by algae to grow like crazy.

Placing your aquarium in direct sunlight will allow the algae to take over within days. Don’t do it.

There are some exceptions

I have to admit there are some exceptions. Of course there are fish who can do well without any light, but these are either blind or have no access to light in their natural habitat.

A great example is this blind cave Mexican tetra you can see on the picture above. Like its name already gives away, it does not have any natural light reaching its natural habitat.

For these fish you can make an exception, but what’s the point? Don’t you want to be able to see and enjoy your aquarium?

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