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Do Fish Need Light At Night (Keep your aquarium natural)

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Many new aquarists are unsure about the ins and outs of aquarium lighting, and a common question I hear people ask is, “Do fish need light at night?”. New tanks often come with a light tube built in, so all logical thought goes out of the window.

It’s not the beginner’s fault as they want to do things just right, and we humans use light at night, so why shouldn’t a fish? Let’s consider why you have a light in your aquarium and whether your fish even need it when it gets dark.

Fish do not need light at night. In the wild, fish are used to periods of daylight and darkness. Too much light and your fish may become stressed and struggle to sleep. Some fish are nocturnal, becoming more active at night and preferring to eat when it’s dark. A tank that is too bright also encourages algae overgrowth.

Do Fish Need Light At Night

When setting up a new tank, remember that your aim is to set up an environment closely resembling a fish’s natural habitat. All living creatures are used to a 24-hour day and night cycle. Dependent on whether the species live in the Northern or Southern Hemisphere will dictate how much light and dark they would expect in every 24-hour cycle.

Unnatural light is a human invention, and constant light will upset a fish’s natural biological clock or circadian rhythms and often induce a stress response.

It has been found that many fish have developed circadian rhythms, which directly impact the production of such things as melatonin, which fish have a higher level of at night and significantly lower during the day. The circadian rhythms control many things, such as a fish’s behavior, level of stress, general health, and even the development of new embryos.

I wrote an article describing how betta fish can see in the dark, which you may find informative.

Do Fish Need Sunlight

Aquariums, in general, have some form of lighting setup built-in. The bulbs are mostly of a standard type and do not emit ultraviolet rays like the sun, so do fish need sunlight, or will a standard light suffice?

While sunlight or ultraviolet light isn’t necessary for fish, small doses are thought to be beneficial. Some aquarists use large doses of UV light to help sterilize their aquarium water as it kills parasites but only use strong UV light before adding fish as it can irritate and burn their skin.

Your main concern should be how much light your fish get throughout the day, and turning the light off at night allows your fish to rest peacefully.


Be aware that some fish may be averse to strong light, so checking any fish’s particular needs is always wise.

Both freshwater fish and marine fish will need around 12 hours of light each day. Not enough light, and you may start to notice their colors fade. Your fish can become lethargic and may stop eating.

If you keep your aquarium lighting cycle natural, you will, in turn, observe the most natural behavior from your fish, both during the day and at night.

If you have a fish tank with access to sunlight, you should ensure your tank is kept away from direct sunlight. When sunlight passes through glass, it becomes magnified and generates more heat, significantly warming the aquarium’s water.

Direct sunlight can also turn your fish tank water green as it can cause an algae bloom. Algae thrive in direct sunlight and can take over a tank quite quickly.

Do Fish Need Sunlight
It is always wise to keep your aquarium away from direct sunlight.

Ultraviolet rays are largely filtered out when passing through glass, so they are unlikely to pass through your window and then your fish tank glass, but during the summer months, a fish tank can become much too hot, far beyond the temperatures your fish will be used to.

Unlike in the wild, where fish can swim deeper to escape the heat, they will have nowhere to go and probably die. Overnight, the tank will cool drastically, and these swings in temperature will not be good for any fish.

If your fish tank can be kept away from direct sunlight and still get enough natural light from a window, an internal light will not be necessary. I prefer to use tubes as they are designed to give the right amount of light, and I will use them in conjunction with natural sunlight.

When fish get enough light, it will help bring out their vivid colors. My aquariums have always been planted, and plants need light to keep them healthy and oxygenated.

Plants absorb carbon dioxide during daylight hours and convert it into oxygen, a process known as photosynthesis that is extremely beneficial to your tank. During the night, your plants will absorb oxygen, but as the nighttime hours are shorter, this will not have much of an impact.

Aquarium water can hold more oxygen when cooler, so too much sunlight can reduce oxygen levels. Another problem caused by too much light is algae growth (Algae Bloom).

Algae in fish tanks can lead to lower oxygen levels and higher carbon dioxide levels, which in turn may cause undue stress to your fish.

Do Fish Need Light To Eat

Most fish don’t need light to eat because they have excellent eyesight in the dark. Mixed with their incredible sense of smell, fish will easily eat in light and dark environments.

Some fish are more nocturnal and prefer to eat at night, relying on their senses and good eyesight. They may enjoy hunting live foods such as aquarium scuds, which will be much easier at night as these fish often rely on stealth.

If you enjoy river or coastal fishing, you will know that nighttime often yields the best results, as it’s when fish feed much more aggressively.

Do Fish Need Darkness To Sleep

Fish don’t need darkness to sleep, but providing them with the most natural cycle will help if you want the best from your fish.

Fish will feel more relaxed in the dark as their instinct to watch for predators or other dangers will be less intense and will help them to sleep better.

Fish are pretty close to the bottom of the food chain in the wild and need to be alert most of the time, so a few hours of darkness will be very welcome. Providing hiding places within your aquarium will also help fish to seek shelter when sleeping.

Depending on the species of fish you own, they may sleep on and off throughout the day and night, so it would be wise to research the natural behavior of the fish you own to understand their individual needs better.

Wrap Up

Lighting is essential in an aquarium for many reasons. It influences the growth of plants and the behavior of the aquarium’s inhabitants and allows us to observe the tank at its best.

Hopefully, by reading this article, you can take away that lighting is not a difficult subject to understand and is often overthought. Yes, lighting can be much more complex, but by following some common sense, you will not harm your fish, and they will be perfectly happy.

If you ask yourself such questions as “Do fish need light at night?” or “Do fish need light to eat?”, ask yourself how they would cope in the wild, and I am confident you will answer your own questions.

Happy Fish Keeping!

Frequently Asked Questions

No fish don’t need total darkness at night, and they wouldn’t necessarily get total darkness in their natural habitat because moonlight can be quite a strong light source.

Aquarium lights are useful and provide a good quality of light which help bring out the colors in your fish and the tank environment, but fish do not need aquarium lights if there is enough ambient light available.

Blue light is commonly used to help with the nocturnal activities of your fish. It is often called moonlight blue as it mimics the blue tone present at nighttime. Fish can sleep fine with this blue light as it is much weaker than daylight, but it is advisable to turn it off at nighttime, even if only for a few hours.

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