Can Aquarium Fish See in The Dark—Do They Need Darkness
By Eddie Waithaka @aquariawise
Updated, 3rd March, 2023.
Among the many odd questions aquarists ask is whether aquarium fish can see in the dark?
Well, the straight and simple answer is NO! But there is a caveat.
Aquarium fish do not see in the dark (at night), but they have pressure-sensitive organs on each side of their bodies that help them sense upsets in the water column and avoid predators.
Having lights-off periods (at the same time) every day is natural and necessary for all fish to thrive. Your finnies will use this time to rest and up their energy levels for the next day.
The recommended aquarium lights-off period is 12 hours a day. Not having these light-off periods will stress your fish.
Perhaps it’s only skittish or nocturnal species, like bristlenose plecos and kuhli loach, that need dim light in the aquarium at night, which you can achieve with moonlight light settings on your LED aquarium lights.
Keep reading for more insight into how fish see and how this affects the the way we light freshwater aquariums.
Can Your Aquarium Fish See at Night
Your fish can see in the aquarium at night if the tank or room lights are on. So, we recommend switching off the lights because the darkness will help your fish sleep.
Use moonlight bulb settings if you must have lights on in your tank at night, and cover it with a blanket or bedsheet to keep your room lights from getting to your aquarium.
How Do Fish See in The Dark (at Night)
Aquarium fish do not see with their eyes in the dark. They have rows of pressure-sensitive organs in lateral lines running down each side of their body called neuromasts, which allow them to locate objects and other animals through movement and pressure changes in the water column.
This ability also helps fish “see” or detect other animals underwater in natural settings, where light penetration is low, or at night in a dark fish tank.
Some tropical fish kept in aquariums, like elephant nose, also have a weak electrical organ on their caudal fin, which they can use to locate food, similar to sharks and electric eels in the wild.
How Do Fish See Food in The Tank at Night (Dark)
Fish depend on their nocturnal ability, smell, and unique sensory organs to locate food at night.
Fish may rely on other senses, such as smell and pressure changes to locate (‘see’) food in a dark tank at night. Some nocturnal fish are adapted to see in low light conditions, including at night.
Catfish and other bottom feeders (more likely to eat at night) may also have barbs or whiskers that help detect food particles at the bottom of the tank, even in the dark.
Besides, most of your fish will be sleeping at night. Only a few species of fish and critters (snails, shrimp) in your tank need to feed in darkness.
Do Aquarium Fish Need Darkness
During periods of darkness, your fish will most likely find a quiet spot within the plants and nap.
Now that we have determined aquarium fish don’t see in the dark (they use neuromasts to detect things instead), the other pressing question would be, do aquarium fish need darkness or should you leave your aquarium lights on at night or 24/7.
Fish require both periods of light and darkness because they need to rest and regain their energy after a whole day of swimming, searching for food, and mates.
For this reason, it is recommended that you leave your aquarium lights on for a maximum of 12 hours (during the day), then switch them off the rest of the time to mimic a day’s light and dark cycle.
Note that if your aquarium is near a window or in a room with adequate natural lighting, you may not need to turn on your aquarium lights at all.
The rays from the sun are enough to light your fish tank during the day (8 to 10 hours), and you do not want excess light (and nutrients) in your tank because it will cause algae to grow.
Please understand that although fish need periods of darkness to rest, their sleeping patterns are not anything like those of human beings.
Aquarium fish can rest at any time, not only at night when it’s dark, and sometimes the only reason they need the said period of darkness is to match their internal circadian rhythm that regulates their sleep and wake cycles, to which all living things are adapted.
Fish like bristlenose pleco and some inverts like snail are more active at night and may use the period of darkness (when you switch off your aquarium lights) to feed.
Do Aquarium Fish Sleep
As I have mentioned, aquarium fish need a period of darkness to mimic a normal day cycle. They need several hours of light to engage in normal-fish business and a few dark hours at night to rest.
So, does this then mean that tropical fish do sleep?
As a matter of fact, YES, aquarium fish do sleep!
If you watch your fish long enough, you may notice they take breaks.
Fish sleep time is characterized by the fish hovering in one place, almost like they’re in a trance.
If you find your fish sleeping at the bottom of the tank by chance, you may think they are sick. Some, like yoyo and kuhli loaches, sleep even in positions that look stuck or dead.
However, don’t expect to catch your fish sleeping with its eyes closed because they do not have eyelids; fish sleep with their eyes open.
You may also never find all your fish sleeping at the same time…
…case in point is when you switch the aquarium lights off!
This is because, for the fish, sleep is any length of time when they take a break from swimming to build up more energy, and this can happen at any time of the day.
It is easy to see your fish sleeping and not realize it is happening because they will still be moving. The reason for this continuous body movement, albeit slow, is for the fish to maintain a constant flow of water past their gills to get proper oxygen levels in their bodies.
One last thing to know is that although fish don’t sleep under a shelter like humans, prey, shy and skittish fish species may prefer to hide under a cover inside the tank while resting to stay hidden from predators.
Do Aquarium Fish Need Darkness to Sleep
Aquarium fish do not need complete darkness to sleep but a consistent and predictable light cycle is necessary to establish a natural day-night rhythm.
Fish, like most animals, have an internal circadian rhythm that regulates their sleep and wake cycles, becoming less active (and sleep) during the nighttime hours, so it’s necessary to darken your fish tank at night.
Mimicking natural light cycles will also promote good health and reduce stress in fish.
Look to provide a consistent and appropriate light cycle for aquarium fish, which involves providing 8-12 hours of light followed by 8-12 hours of darkness (and sleep).
Happy fish keeping!