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Do Aquarium Fish Need Darkness?

This seems to be an ongoing discussion in the aquarium community. I know my view on this has changed over the years and yours probably will too after reading this article. It is always good to hear what other, more experienced hobbyists have to say about things like this in order to help you make the right decisions for the well-being of your fish.

Do aquarium fish need dark? Yes, some aquarium fish that are nocturnal will require a period of darkness to eat and to be healthy and happy in that environment. Most other aquarium fish will do fine with a combination of light and dark. The exception are fish that are skittish and require some dark otherwise they become stressed and eventually die. 

I think it’s important to understand the relationship that lights and darkness have with your aquarium. Yes, as mentioned above, some nocturnal fish will need periods of darkness or they just won’t survive for very long. But do all fish need it? We as humans re-energize when it is dark and live and flourish in the light. Let’s talk a bit about the different sides of aquarium fish needing dark or not.


What Kind of Aquarium Fish Need Darkness?

As I previously mentioned, the type of fish requiring darkness include nocturnal fish and skittish fish. What about regular aquarium fish, though? Honestly, the only way you can find out how a fish will do is to test it out. If you left the lights on 24/7 for a week, how would your fish act versus if you had a light on for 8 hours a day and gave some solid darkness to the inhabitants?

I feel there would be a noticeable difference in the fish. Fish that are exposed to 24/7 light would be a bit more on edge. When it’s always light, there just isn’t any down time for the fish. When it is dark, I really believe your fish will feel safer and will reflect that in how they act in the fish tank over time. Fish are always on the guard from predators and having a period of darkness where they can go and rest for a while should help them feel more secure.

How Long Do Aquarium Fish Need Darkness?

When deciding how long to leave the fish tank lights off, consider these things. How much indirect light does the tank get, is there a problem with algae? What is your schedule? We know that having darkness is good for the health of nocturnal and skittish fish and for every other kind of fish or tank it’s also a good thing.

When fish are in an aquarium in a home there will always be other sources of light aside from the tank’s light. You can consider that when you decide how long to leave the lights on. Contrary to popular belief, fish don’t need 8 to 12 hours a day of bright aquarium light.

If your tank sits in sunlight or a very bright room, you might have a bit of an algae issue. One thing you don’t want to do is feed the algae with too much bright light. You will spend most of your times cleaning the inside of the tank. Setting your lights to be on when you will get to enjoy the aquarium the most is the best idea. You don’t need the lights to be on when you are away from home at work or elsewhere.

I recommend trying a 6-hour window when the lights will be on. This allows plenty of darkness when it’s actually dark out and gives the fish some time with indirect light. Unless you have the tank in a basement where there is no window to the outside, then you might want to have a lamp or something like that come on for a bit as well.

The point I am trying to get at here is to think more of how long to have the lights on instead of how long to keep them off. You don’t need them on for very long and the time they are on you should be able to enjoy the tank. 

Up next, I will answer some related questions I found online that I felt would be helpful in regards to this topic.


Should I Turn My Fish Tank Light Off at Night?

I think it’s safe to say that, having the previous information in this article, you know that yes, you should turn your light off at night. Some fish thrive in the dark and the rest will want to rest. Plus, do you really want bright aquarium lights on when you’re not enjoying the tank anyways?

koi fish lit up over water

Can Fish Swim in the Dark?

Yes, definitely. I am sure you have seen your fish moving around in the dark. They probably see better in the dark than we do! Your fish will be very used to its surroundings and will have no problem navigating the tank in the dark.


Do Fish Sleep When the Lights are Off?

If that’s what your fish want to do then yes, they will sleep when the lights are off. If you are sneaky, you can find them nestling among plants and other aquarium objects to relax. Some will even lay right on the bottom. This of course depends on the species. For example, you can expect a cory catfish to be found sleeping on the substrate however, a neon tetra might just nestle in some live plants.


When Should I Turn My Aquarium Light Off?

There isn’t an exact perfect time when your tank lights need to be turned off. When the lights do go out, I would think it should be when you’re finished viewing for the day. Make sense?

I will tell you what I do. I have all of my tanks lights on timers and they go off at 9PM every day. I selected 9PM because that is usually when I go to bed during the week. Most nights, I stay up later and the lights go off while I am still in the room. That’s okay, as its great to see the lights working the way they should.

I also time when the lights turn on based on when they shut off. Currently, I have the lights come on at 12 noon every day. I do have a bit of algae so am thinking of setting the timers to 1PM instead, as this is midday and the tanks get a ton of indirect sunlight anyways.


Do Fish Like Blue Light at Night?

When I turn the blue light on, my LED aquarium lights my fish keep swimming around. I usually don’t turn the blue light on for more than an hour. I also don’t turn it on every day. I don’t think your aquarium fish mind having the blue light on for a while however, I would watch their behaviour and if they seem to try to hide when the blue light is on, I would take that as a sign that they don’t like it.



In conclusion I just want to reiterate that most fish, whether they need darkness or not, will benefit from it. Your fish community will be much healthier when they have a period of darkness. Plus, why would you want lights on all night when you’re not even up to enjoy them?

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