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Do Fish Like the Dark? For Health, Behavior and More

Humans have always tried to avoid darkness, hence the rapid and continuous advancement in the light industry. Light is unquestionably essential for all species on Earth, not only humans — we can thrive in terms of warmth, growth, and well-being. However, certain living creatures prefer darkness for survival reasons — are fish among them?

Fish don’t like the dark because when there is no light to see, they dart around the tank, colliding with the walls, ornaments and can even become tangled in the plants. Fish need light to determine whether it is night or day, and they’re scared of the sudden switch from darkness to light.

In this article, I’ll take you through how fish react in the dark, along with explaining how many hours of darkness fish need as well as what happens if you leave them in the dark. 

How Do Fish React in the Dark?

Just like how you and I are aware of day and night, the sense to work in the day and rest at night is similar for fish. Fish engage in various activities like swimming for food and copulation when it’s light. 

After these activities, fish need time to rest. 

Fish will slow down their pace, rest, sleep, and sometimes hide in the dark. If the darkness has been gradual (as the sun sets), they will naturally ease into this restful period. But, if you suddenly darken the room, they may react poorly and become confused. 

To ease yourself of stress and to make life easy for your fish, you should apply the same typical day routine you use with them.

Use the same light and dark cycle for your fish as you put on the light when you are awake and switch it off when you are about to sleep. As this becomes a routine, your fish adapts and maximizes the time to rest as well.

An experiment conducted by Anika Bruning highlights that light delays the sexual maturation of fish. The result of her laboratory experiments suggested that the reproductive mechanisms of fish may likewise react to deficient levels of light at night. 

However, there appears to be a particular light-sensitive time in the year in which this is the case.

Rivers, streams, and clear lakes are the most popular sources of community aquarium fish. The bright sunlight and warm rays of rainforest weather are second to these fish. However, there must be darker areas to accommodate the fish that prefer to hide from the light.

The receptivity of fish to light varies greatly depending on the species and developmental stage. Both the eye and the pineal gland have photoreceptor cells.

In his explanation of how fish see in the dark underwater, EDDIE WAITHAKA said that “fish have rows of pressure-sensitive organs in lateral lines — running down each side of their body.”

These lines are called ‘neuromasts .’ They allow fish to sense nearby animals from pressure changes in the water column. 

The structure and function of fish eyes are pretty similar to that of human eyes. The most significant difference is that fish lenses are spherical rather than compressed, focusing better underwater.

Fish may be better at seeing in low light, such as the bottom of a muddy lake, or brilliant light, like those living on sunny coral reefs, depending on their lifestyle. Some fish can even perceive ultraviolet and polarized light, which humans are unable to do.

How Many Hours of Darkness Do Fish Need?

Most tropical aquariums require at least 12 hours of darkness every day, whereas cold-water aquariums require more, meaning you should only illuminate fish tanks for 8–12 hours each day.

Fish living in aquariums require 12-16 hours of darkness every day. This is due to the fact they need 8-12 hours of light a day to thrive. Light timers and alarms are the most straightforward and successful approaches to regulating your fish’s night and day cycle.

Consistent light cycles that mirror daytime and nighttime alleviates stress and replicate natural settings. On the other hand, tank placement plays a significant role in this, and if your tank is placed incorrectly, it can affect its success.

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. By doing this, you can make your proposed day and night cycles applicable to your fish.

A survey proved that many fish lovers believe that a maximum of 10 hours of light is enough for the fish while they stay in the dark throughout the day. 

Many factors must be considered when looking at the fair number of light and darkness for fish, for example:

  • The fish species you have or you want in your aquarium.
  • The numbers and plant types you will have in your tank.
  • The ambient location lighting.

If you don’t have any live plants in your tank, you can get away with utilizing less light. On the other hand, some fish may not appreciate the reduced lighting hours, and sleep is essential for these fish because it promotes improved health and illness resistance.

Algae can develop faster in a tank that is lighted for more hours of the day — this can make your tank look unclean, and if you have natural plants in your tank, algae may be able to outcompete them. Large algal bloom can also cause oxygen deficiency which can kill your fish.

What Happens if You Leave a Fish in the Dark?

Fish love to rest to regain strength, so they await darkness. However, they also need light, in the same way we do.

If you leave a fish in the dark, it may begin to lose its color and, in some cases, turn white. This happens because their pigments depend on light. These pigments consist of cells that reflect light and produce color. Without it, they will soon fade.

According to ThoughtCo, fish have ‘chromatophores. These are cells that produce the pigments responsible for coloration or reflect light. 

The pigments in a fish’s cells, the number of pigment molecules, and whether the pigment is bundled in the cell or distributed throughout the cytoplasm contribute to the fish’s color.

Why Shouldn’t You Use Sunlight for Your Fish?

You shouldn’t use sunlight for your fish because you will be left with very little control over it or its consequences on your fish tank. Too much sunlight can have adverse effects, such as algal bloom and an increase in tank temperature.

One of the most severe issues with sunlight is the possibility of an algal bloom. An algae bloom will not only diminish the visibility of your tank but will also harm your fish. 

Algae blooms frequently cause bacterial infections, which can kill both the fish in your tank and the plants.

The effect on temperature is another issue that you need to put into consideration. If your fish’s tank is exposed to the sun, the temperature may become uncomfortable or dangerously high. 

An increase of just a couple of degrees could be enough to stress out your fish and perhaps kill them.

Different fish require different temperatures, so examine each variety of fish you have and intend to add to your tank to ensure they can coexist at the same temperature.

Using Artificial Light

Artificial light can sometimes cause your aquarium to reflect on itself. As a result, your fish may become aggressive, believing that its territory is being threatened.

If your fish is behaving aggressively, you may need to purchase a weaker light or find a means to block the reflection from happening.

Also, if the room where your fish is is entirely dark, they won’t do well in the long term. Fish are not picky about the type of light they receive, which is a good thing. As long as your illumination does not cause the water to warm up, you are good to go.

Do Fish Sleep Better in the Dark?

Bart Sprenkels highlights that aquarium fish do not need light, and you should turn off the light during the night.

Fish get more comfortable and sleep better in the dark, so it’s common for them to find a peaceful location where they can rest. You’ll likely see your fish seeking shelter within your aquarium decorations to slumber when the room is dark.

To allow your fish to sleep at night, you need to give them enough darkness. Sleep is highly crucial to your fish’s long-term health. 

Your fish may stop eating or develop ailments if they don’t get enough sleep.

Make an effort to provide a comfortable environment for your fish if you want them to live a happy and healthy life. 

For example, adding pebbles, branches, leaves, and moss to your fish tank. Fish are territorial; thus, they like to sleep under or around objects that provide them with a sense of security and protection.

Like humans, fish don’t sleep only when the environment is dark; they also sleep anytime they are tired and want to regain strength.

The other reason fish require darkness is to mimic the daily cycle to which all living things, including fish, are evolved.

Some fish require periods of darkness to sleep; they may become irritable or stop feeding. Excessive light stresses many fish species, causing them to struggle with one another.

Fish, like humans, require sleep to re-energize their bodies and maintain healthy immunological function. They lose their ability to fight illnesses, and their metabolism slows if they don’t get enough sleep.

The fish determine the amount of sleep required. For example, some goldfish take a nap in the afternoon, while others don’t sleep until evening. 

How Do You Know if Your Fish Is Sleeping?

Fish don’t have eyelids, so you might wonder how to know when your fish is sleeping.

You can tell when your fish is sleeping by observing their motions. Fish eyes remain open, and they don’t just stay in a place while sleeping; they move, although the movement is slow. But sometimes, fish will hover in one position during these sleep times, almost as if they are in a trance. 

The reason for the fish’s persistent, albeit slow, body movement is to keep a constant flow of water past their gills to maintain proper oxygen levels in their bodies.

Because fish use less oxygen while sleeping, their gill motions will be slower. 

Fish can sleep in a variety of positions, and some prefer to rest along the edge of their tank. Prey, shy, and nervous fish species may like to relax under a cover inside the tank to avoid being seen by predators.

Although, they may not appear to be sleeping in the same way that mammals or fluffy pets do. Fish require rest periods in which their activity and metabolism are reduced. Many fish remain vigilant to danger; therefore, they have some brain activity.

If you have a fish migrating, spawning, or caring for her babies, she may choose to hold off on sleep until that time has passed.

Final Thoughts 

Darkness has its usefulness for fish. It helps them sleep better when they’re tired. However, you can’t keep their tank in the dark all the time, and you will need to have some source of light.

It is also crucial that you avoid using sunlight as illumination for your fish tank — sunlight isn’t a controlled source, and it can cause problems for your fish.