Can Aquarium Fish See in the Dark?

For the beginner aquarist, it might not be obvious whether aquarium fish are capable of seeing in the dark or not. 

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Aquarium fish actually can’t see anything when the aquarium is pitch black. However, they can orient themselves in a similar way as bats do.  While bats use sound waves to get a better sense of their position relative to their surroundings, aquarium fish can interpret water waves in a similar way.

They have so-called neuromasts, which are lateral lines on their body that help them sense pressure around their body. As the pressure changes, they get a good sense of the position of other animals or objects around them.

This allows them to be aware of most of the things around them while they are swimming around in the tank. Some tropical fish species even have an electrical organ that comes in handy when looking for food. They have developed this particular organ on their caudal fin.

What do Fish do in the Dark?

For fish species, it is not a problem at all to orient themselves in the dark. They are pretty good at it, as they are not so much reliant on their eyesight. A much more reliable way for them to find food, shelter and swim around in general is to sense pressure changes in water with their sensitive organs.

Those are the lateral lines you can see on your fish. They use those to sense nearby fish with great precision. However, fish species that are not nocturnal are most likely going to spend most of their time sleeping in the dark.

The aquarium being in darkness is a sign for them that it is time to take some rest. Just like us, humans, they also have an internal clock that tells them when to sleep. Not only that, but darkness also makes it easier for them to fall asleep, just like for us.

Do Fish Sleep at Night?

Since every fish has once lived outside in nature, they need a regular day and night cycle, just like us. If they don’t get it, they are simply not able to rest. During the night, they can get some time off and sleep until they are well-rested.

Fish don’t only sleep but they also take naps a couple of times a day. If you see your fish levitating and spending time pretty much being inactive, then they are most likely asleep. Meanwhile, maybe they would close their eyes if they could but they are not able to.

They have no eyelids so all they can do is to enter a trance state and shut most of their body off. Most fish are not going to sleep all night long. They sleep just as much as they need to until they are full of energy again.

What characterizes typical sleep for fish is slow body movement. The reason behind this is they still need to get oxygen through their gills, which are constantly moving.

Do Fish Sleep During the Day?

Most fish species that people keep at home are used to a regular cycle and sleep pretty much when we do, during the night. There are, of course, exceptions which we call nocturnal fish species that prefer to be active at night.

Fish can also get exhausted during the day and can resort to short naps in order to muster some extra energy. If you notice your fish being inactive throughout the day, floating around and doing nothing in particular, then it is most likely having a nap.

Can You Keep Light on at Night?

It is more natural for fish to feel the natural day and night cycle throughout the day. Even nocturnal fish are used to the two main phases of the day. Even if you keep your pet fish in a dark room, you need to ensure the lights are on for about 12 hours.

Then you need to switch them off and repeat this every day on a consistent basis to maintain a healthy cycle. It is even more natural to keep track of the day cycle outside and simulate the same inside the aquarium. After all, daylight hours vary and depending on the fish species you keep at home, they might be sensitive to it.

This will require you to do some research in order to find out the healthiest cycle for your pet fish during any given season. It is important to understand that aquarium lights are not only about aesthetics. Although there are plenty of awesome lights to choose from, you need to use them wisely in the tank.

Which Aquarium Fish are Active at Night?

Although the overwhelming majority of fish species are diurnal, there are some species that are nocturnal as well. They spend most of their time swimming around and looking for food at night. During the day, they usually find a shelter to hide at in the aquarium and try to get some sleep.

Numerous beautiful Catfish, Kinfe fish, Plecos and Loaches fall into this category. Be careful when keeping diurnal fish with nocturnal ones because they can interfere with each other’s lifestyle.

Wrap Up

To conclude, aquarium fish species can’t see in the dark. These little pets didn’t develop such a sophisticated visual organ as us, humans did. After all, we also can’t see much in the dark. Fish rely more on sensing the water pressure around them.

There are not that many nocturnal fish species either, but you can find some exciting ones for sure if you are interested in those. Whether you have diurnal or nocturnal fish species at home, it is important to simulate the natural day cycle for them.

After all, all of them have initially lived in the wild before we discovered them and started to keep them at home. Setting up the tank in a way that mirrors their natural environment is very important. This means that you need to handle the lights appropriately and keep them on for a maximum of 12 hours.

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