Can Fish Live in Dark and What Do They Do at Night

Keep fish for a few months, and chances are that darkness will descend upon your tank. Aquarium lighting breaks, electricity goes off, and fishkeepers need vacations.

We’ve all been there.

But is it safe to leave the aquarium in darkness, and for how long?

Fish could survive in the dark for a few days, but prolonged exposure will make them suffer, wither, and eventually die. Adequate lighting helps fish and plants thrive.  Regular periods of light exposure simulate the day-and-night cycle and help fish regulate their sleep and overall behavior. Live plants also need light to photosynthesize.

While lighting is a complex topic, especially when you have live plants, here are some basics to get you started on the right foot.

Do Fish Need Aquarium Light?

Fish need light to be healthy and happy.

It could come from the overhead lamp, but a designated aquarium light is a much better solution. It enhances the aesthetic impact of a fish tank manifold by casting direct light on its setup.

What’s more, most modern lights have at least three degrees of intensity that help with plant growth, algae control, and overall water quality.

While lights break and everyone travels, try your best to provide regular light exposure.

There are timers that can turn on and off the light at specific times. Aquarium lights with built-in or installed timers are the best solutions, even when you are at home, to create an immaculate light schedule. 

Here’s a potential caveat.

If you have to travel and you are yet to procure a timer, it is better to leave the tank light off, given that the aquarium is in a room with windows. Assuming that no direct sunlight reaches the aquarium, keep the curtains open, and the light of the day will help your fish with their day-night cycles.

Of course, asking a friend to turn the light on and off is another option.

Why Sunlight Is Bad for Aquariums

Exposing your tank to direct sunlight is not a good idea because it is highly likely to produce algae and warmth. Both can have a detrimental impact on flora and fauna of your aquarium.

Algae, besides being aesthetically unpleasing, can quickly cover the leaves of your water plants. Leaves covered in algae cannot photosynthesize nor absorb nutrients from the water column. While the infested plants won’t die right away, algae impede their growth and reproduction.

But the immediate danger is warmth.

Direct sunlight will likely cause fluctuations in the temperature of the water column. While the sun is hitting the aquarium directly, the water will warm up. On hot, bright days, the increase can be too much for some species to handle.

Respectively, when there is no sun, the temperature will drop. Such fluctuations stress the fish. Most decorative fish are bred in tanks with stable temperatures.

Direct sunlight is the worst source of light for a fish tank.

The overhead room light is better as it won’t change the temperature of your aquarium. Still, the best option is a dedicated aquarium light.

Can Fish See in Darkness?

Aquarium fish cannot see in complete darkness.

While there are deep-water ocean species that can, practically all aquarium fish originate from relatively shallow waters, where light penetrates.

Fish need light to orient themselves, even though the lateral line organ also helps in that regard.

But vision is crucial.

Most fish have keen eyes. They use them to find food, to move, to chase, and so forth. Eye shape and structure differ among species. Some are more adept at perceiving color, others detect movement better, yet others see well in low light.

Now, at night, when the lights of the tank are out, the darkness rarely is complete. Usually, some ambient light comes from outside. This is the time when many nocturnal species become active and start foraging for food.

What Do Fish Do at Night?

Most fish sleep at night.

They use darkness for restoration, even though fish don’t have eyelids and cannot close their eyes.

Wicked, eh?

It gets even stranger as all non-bottom dwellers float while resting. They move their fins just enough to stay at the chosen resting spot.

This sounds unimaginable for us, humans, who close our eyes and are mostly paralyzed while sleeping, but life underwater is different.

Fish definitely enter a state of deep rest. Their metabolism slows down, and they barely move. Virtually all fish look for a secure spot to rest, be it under a rock, in a cave, among the cover of plants, or even in the shadow of the submersible filter.

Of course, all the above applies to diurnal species.

Nocturnal fish usually hide (in relative darkness) during the day and rummage for food once the lights are out. Species like loaches and plecos prefer shady places — give them caves, if you ask me — while the lights are on and get the party started soon after darkness descends.

If you have many nocturnal species, it may be a good idea to feed them when the tank is dark.

How to Observe Fish at Night

How do I know what fish do at night?

I’ve used a small torchlight to spy on my tank in darkness more than once. To not bother the fish, it’s better to use a torch with a focused beam instead of a cell phone. Most smartphones have decent light options, but their beams are rather dispersed and don’t provide enough illumination at the back of the tank.

The scattered light is likely to wake up more fish than you’d ideally like as well.

To see how and where fish sleep or roam around, turn off the aquarium light — and the overhead lamp in the room — as you’d normally do and wait for 15-20 minutes. Then use a torchlight to cast a light on them.

Flash the light for 10-20 seconds at a time, turn it off, wait for 20 seconds, then go again. Choose in advance which part of the tank you want to observe or which fish you want to find. For best results, I place the torchlight on the tank hood, so the light penetrates from top to bottom.

It is important not to overdo it, both in terms of frequency and duration. A few minutes once a week is fine, but more than that can be stressful. Imagine how you’d feel if someone was to cast a light in to your (open) eyes while sleeping.

It is OK to check on the dark tank every now and again as a special window into the life of your aquatic friends.

Also, do it soon after you’ve turned off the lights. The upside is that if your light wakes some of the fish up, they still will have enough nighttime ahead for sufficient rest.

Fish go to sleep immediately after dark, so you will get a good view of who hides where and who scurries for food.

Conclusion

Don’t keep your fish in the dark for too long, especially in the complete darkness of a windowless room. 

The best solution is to equip your tank with a dedicated lamp with a timer. Many lights come with one, but you can buy them separately as well.

Light is a crucial element of your tank setup and promotes health and healthy behavior among its inhabitants. While a few days without light — due to a power outage or broken light — aren’t the end of the world, prolonged exposure to darkness will stress and harm your fish and plants.