Can Aquarium Fish See in The Dark—Do They Need Darkness

By Eddie Waithaka @aquariawise

Can Aquarium Fish See in The Dark—Do They Need Darkness

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, whether betta, goldfish, guppies or otherwise don’t exactly see in the dark, at least not with their eyes.

Instead, they have rows of pressure-sensitive organs in lateral lines running down each side of their body called neuromasts, which allow them to sense nearby animals from pressure changes in the water column.

This ability then explains how fish “see” or detect other animals underwater, where light penetration is low or at night in the dark.

A few tropical fish kept in aquariums like elephant nose also have a weak electrical organ on their caudal fin, which they use to locate food in the same way sharks and electric eels do in the wild.

Keep reading for more insight on how fish see, and how this affects the way we light freshwater aquariums.

Do Aquarium Fish Need Darkness?

Now that we have determined aquarium fish don’t see in the dark and use neuromasts to detect things instead, the other pressing question would be, do aquarium fish need darkness then, and should you leave your aquarium lights on 24/7 for the fish to feed and play even at night.

Like human beings, most 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, then switch them off the rest of the time to mimic a normal day’s light and dark cycle.

During this period of darkness, your fish will most likely find a quiet spot within the decorations and nap.

However, it’s also good to 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 anytime, not only at night when it’s dark. Hence, the other reason fish need the said period of darkness is to simulate the diurnal-cycle, which all living things including animals are adapted to.

Moreover, fish like bristlenose pleco and some inverts like snail are more active at night, hence take the advantage of the period of darkness when you switch of the your aquarium lights to feed.

Do Aquarium Fish Sleep?

As I have mentioned before, aquarium fish need a period of darkness to mimic a normal day cycle like they are used to and for them to get some time off.

So, does this then mean that tropical fish do sleep?

As a matter of fact, it does!

If you watch your fish long enough, you may notice that they do take breaks. These periods of sleep are characterized by the fish hovering in one place, almost like they’re in a trance.

Even so, don’t expect to see your fish sleeping with its eyes closed because they do not have eyelids; fish sleep with their eyes open.

Also, you will never find all your fish sleeping at the same time, case in point, when you switch 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 day.

That said, it easy to see your fish sleeping and not realize that’s what ’s 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 as this will help them stay out of a predator’s sight.

Can You Leave The Lights on in a Fish Tank at Night (or 24/7)?

Im sure I have already answered this question in part above, but I think it’s also good I give it a little more context for better understanding.

See, like all animals in the world, nocturnal or otherwise, aquarium fish adapt to a period of light followed by a period of darkness, so it’s not recommended to leave aquarium lights on at night, leave alone 24/7.

Most tropical fish have evolved under conditions that provide roughly 12 hours of light a day, so logic suggests that an aquarium with fish will most likely need a combination of ambient and aquarium lighting for about half a day.

Even so, cold-water species such as goldfish, minnows, ricefish, and zebra danios are from temperate climate zones where the daylight hours vary according to the season. For these fish, you therefore might need to vary the amount of light in your tank over the year at between 8 and 12 hours of light a day; 10 light hours is a reasonable median.

When keeping cichlids from South America, tetras, angelfish, discus, and other tropical fish from the Amazon river basin, it’s also advisable to have lesser hours of light because the fish are used to dark water conditions in the wild.

Overall, try as much as possible to match the length of aquarium light hours to what the species you are keeping experiences in the wild.

Having said that, aquarium lighting is both a design feature and a practical necessity. The illumination of a lighted tank makes a beautiful element in a room, which you may want to keep going longer, particularly if you hold evening parties at home.

In such situations, consider getting a set of LED moonlight aquarium bulbs, which are bright enough to illuminate your tank for aesthetics, but with a blue light mode that is dark (compared to normal white light) that does not disrupt your fish sleeping patterns.

Lastly, one of the biggest obstacles to maintaining a uniform period of aquarium lighting is that aquarists find it difficult to turn the lights on and off at the same time each day. Fortunately, there is an inexpensive way to remedy this problem.

Purchase an on-off timer and plug the lighting on it, then set on and off times to obtain the desired period of light each day.

That’s all from this article.

If you are located in the USA (more so Florida) and are looking to buy tropical (freshwater ) mentioned in this or any other post (and more), check out Consolidated Fish Farms Inc.

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