Can Aquarium Fish see in the Dark?

Aquarium fish have eyes! You may have wondered how your aquarium fish manages to move around in the dark. So, do they see in the dark?

Whether it is goldfish, angelfish, guppies, tangs, etc, aquarium fish don’t exactly see in the dark. But how do they manage to move around or eat at the night?

Aquarium fish bears rows of pressure organs that are sensitive. They have lateral lines that run down each side of their body. These lateral lines are referred to as neuromasts. They enable the aquarium fish to sense danger or nearby animals from the pressure changes in the water column. This is how this type of fish manage to locate food in the same way electric eels and sharks do in the wild.

Do These Fish Need Darkness?

Since you now know that aquarium fish don’t see in the dark but rather use their neuromasts to detect things, the focus now shifts too; is there any essence of darkness to aquarium fish? Keep reading, all of this is covered below.

Fish require both periods of light and darkness, just like human beings. After a whole day of swimming, searching for food and mates, aquarium fish need proper rest to regain their energy lost. This is why you ought to leave your aquarium lights on for a maximum of twelve hours and then switch them off the rest of the time. This facilitates the mimicking of the normal day’s light and dark cycle. Your fish will use the period of darkness to locate a quiet spot within the decorations and sleep. Since the aquarium fish can rest at any point in the day, the period of darkness helps to stimulate their diurnal cycle. Some inverts such as snail and more fish-like bristlenose pleco tend to be more active at night. Therefore, this makes the most of the period of darkness whenever the aquarium lights are switched off to feed.

Do Aquarium Fish Need a Light?

Just like most other life on earth, aquarium fish respond to the same circadian rhythms. For outdoor systems, you should try not to blind your fish overnight with underwater light. Mimicking the natural outdoor cycle is ideal for your fish in the lowest stress environment. You should enforce a standard period of light and darkness in case you cannot keep up with the seasonal changes.

Fish Vision

Fish eyes correspond to human eyes in terms of structure and function. The difference is that fish lenses are spherical and thus allows them to focus better underwater. Aquarium fish possess similar cone and rod cells suitable for black or light and color vision. 

Available Lights

Your aquarium shouldn’t be having the bulking, burning halogen lights. LED lights are very cheap, easy, and the standard for most aquariums. They are available in a variety of spectrums and colors that best suits your fish needs. 

An aquarium with already built lights on the hood enables you to swap them if desired. A lot of hoods have a clear plastic panel with a capacity to host a couple of different lights. Any plans to have some corals in your tank will force you to provide a UV-rich spectrum that will promote significant growth of the cohabitating zooxanthellae.


 Can You Leave The Lights on in a Fish Tank at Night (or 24/7)? How long should your lights stay on? You should be able to match your outdoor light and dark cycles. For coral tanks that are much distanced from their natural habitats, the case will be different. Extensive research of the origin of your corals will help you match their wild light cycles which will, in turn, provide the best growth.

Always run away from the idea of placing an indoor fish tank near a window to match your cycles as it is not a great idea. Supper sunny days will heat small volumes of water at a great speed and hurt your fish if they fail to compensate. Natural and UV light are also known to trigger algae blooms which are not necessarily a health issue, but an eyesore. 

Fancier LED lights have programming functions that allow you to set your tank lighting, you can have it go on or off automatically. Many of them contain the ‘sunrise’ and ‘sunset’ feature. The other thing that can have you set your tank up and be finished is the simple electronics timer, or know as the smart plug.

A light period of 12-16 hours and a dark period of 12-18 hours is recommended for indoor aquariums that do not contain corals. Different fish will like different dark and light periods. Fish will sleep and a slow increase in light is far much better than a sudden switch.

Do Aquarium Fish Sleep?

There is no debate about this. Aquarium fish do sleep. The aquarium fish require rest periods to reduce their activity and metabolism. This is way different from humans or fluffy pets. Since the fish need to stay alert to danger in the low energy state, there is still some brain activity. Since they do not have eyelids, they cannot close off their sight.

With that being said, let us look at how fish sleep in a home aquarium!

Have you ever seen an aquarium fish sleeping? Although it is very easy to understand when humans sleep, it is slightly difficult when it comes to fish. So, what should you know about fish sleeping? Do not worry, it’s all explained below!

The Sleep Stages of an Aquarium Fish

Despite fish having different anatomy as compared to that with mammals, they do also require rest. Fish have two different methods of sleeping. These methods consist of alert rest and actual sleep. During the alert rest, the brain sleeps but the senses are active. In this type of sleep, they are always alert to be prepared for any dangerous situation. 

The second type, the actual sleep, is where the fish suspends all activities and the body is completely on standby!

Aquarium Fish… Sleeping?

A sleeping fish cannot be recognized by observing the eyes. We already know why! Fish lack eyelids and thus never have their eyes closed.

Have a look at the pace of your aquarium fish. If it is phlegmatic and the fish appears to be almost stopped, then your fish may be sleeping. The fish may also be sleeping if they become less sensitive to external stimuli. Try feeding your goldfish or fish when it has stopped at the bottom or behind a decoration! You will notice that the fish will not run towards the food. The slow movements and a reduced heart rate shows that the metabolism has decreased and as a consequence, the fish is saving energy to rest the body.

Generally, you will find all the fish that dwell in home aquariums lying motionless or near the surface of the water. You are advised never to disturb them while they are sleeping. This is because you may frighten them and cause them severe stress!

How Long do Fish Sleep?

The type of species and the environment will determine how long your fish will sleep. Some will prefer to take very short and frequent naps both during the day and during the night. Other fish will prefer to sleep only at night, while others are diurnal. There are some species of fish that fail to sleep at certain times of their lives. This is for instance when they are migrating or taking care of their young ones.

Fish that live inside the aquarium tend to synchronize their biorhythm with the lines of switching on and off the light of the tank, and particularly of the family takes care of them. This way, you can find your fish sleeping at night after you have turned off the lights of the tank.

To sum it up, you will never find all your fish in the aquarium sleeping at the same time. For example, when you switch off the aquarium lights! Sleep refers to any period when they take a break from swimming to build up more energy. This can happen at any point in the day. Besides, it’s never easy to see fish sleeping as they will always be in motion. The reason behind this continuous slow body movement is to ensure a constant flow of water past their gills so that they can get sufficient oxygen.

In Conclusion

We have seen that fish do not exactly see in the dark, they remain sensitive to the external stimuli with the help of neuromasts. Fish need both periods of light and darkness for their survival in the aquarium. How you regulate this depends on you. Since fish sleep, you should be wary not to disturb them so they don’t obtain additional stress.