How Many Hours of Darkness do Fish Need?

Fish are one of the most popular pets in the USA and can outnumber cats and dogs. Unlike these, fish are unable to fend for themselves, so they do need care and attention. You can find different types of aquariums, be it tropical or cold water. What some tank owners don’t know in the beginning is that fish have a day and night cycle just as humans do.

This leads to a common question that tank owners ask often. So, how many hours of darkness do fish need? Most tropical tanks need a minimum of 12 hours of darkness per day, while cold-water tanks need more. However, it is easier to understand when you see how much light each tank requires that determines the number of hours of darkness.

Here, we will see how much light your tanks need, and then you have a very simple calculation to determine the number of hour’s darkness your fish require. You will also see, this can dictate where your tank sits, and when you need to turn your tank light off.

How Light Affects Your Fish

One good thing with fish is they are not too fussy with the kind of light they receive. As long as your lighting doesn’t warm the water, then you can use any light such as LEDs, fluorescents or incandescent (these do produce heat).

Because fish are cold-blooded, warm water affects them when it is outside their desired range. Different fish do require different temperatures, so when looking to stock your tank, be sure to check each species of fish you want, and make sure they can cohabit in the same temps.

Another area where some types of fish benefit from sufficient tank lighting are their color. When there is a bright light, it darkens the color pigments in the fish.

However, there is one downside to all this light, and it has the name algae.

How Aquarium Light Affects Algae

Alga is a natural part of any aquarium, and it isn’t merely the sign of a tank that isn’t clean. Algae won’t thrive on light alone and will need nutrients just the same as any other living organism.

In many cases, you see this growth in a new aquarium before it cycles. As soon as aquarium plants situate themselves and begin growing, then they can compete with the algae for the nutrients. Luckily, plants have a larger nutrient requirement, so they often come out on top.

If you find your algae problem isn’t going, it may be because you are using water from the faucet, and the algae are feeding on the tap water nutrients.

Too much light can also be a reason this algae thrives. You can reduce the amount of light to your tank, yet you don’t want to do this at the detriment of your fish, or your plants.

How Light Affects Aquarium Plants

Many tank owners focus on their fish and overlook how lighting affects their aquarium plants. Even though these plants are submerged underwater, they still require light and darkness. Tropical plants need more light for them to carry out photosynthesis.

Additionally, plants that grow close to the surface need more light as they would in their natural habitat. Plants need as much consideration as the fish that will be included in the tank, as these can affect the lighting system and the amount of light the tank requires. Aquarium plants that have red colors in the leaves need lots more light.

Plants perform in much the same way as regular plants during the day. They take carbon dioxide from the water, photosynthesize, and release oxygen. However, at night, they carry out the opposite, and in some instances, this can pose problems for fish.

In the darkness, plants leech oxygen from the water for their survival. As a result, they release carbon dioxide into the water. If there isn’t sufficient aeration during these periods, then the oxygen reserves in the water deplete, and fish begin suffocating.

Some tips for light exposure when your aquarium has live plants are:

  • The suggested light exposure for live aquarium plants is 12 hours of light each day. However, the specific amount can vary depending on the setup of your aquarium and the plant species that are living in it.
  • Be aware that there is less light exposure needed if your aquarium has no live plants.
  • Make sure that you match the type of live plants in your aquarium to the type of fish. For instance, tropical fish should be housed with tropical plants. The different types of fish and plants generally rely on the same amount of light exposure.
  • After introducing new live plants to your aquarium, provide more light exposure so they can take root and grow faster.

Darkness and Fish

Darkness is critical for fish. Like humans, fish need to rest their bodies. It is here we begin to see the differences in the hours of light, and thus the hours of darkness fish need daily.

Most aquariums can thrive with around eight or twelve hours of light per day. The remainder of the time, in theory, will be in darkness. However, tank placement plays a massive part in this and can affect the success or failure of your tank if it is in the wrong location.  

While many fish enthusiasts reckon ten hours of light per day is ideal, when you look at the range of between 8 and 12, it is a broad range. To narrow this down, many factors go into this.

  • The fish species you have, or you want in your aquarium
  • The numbers and plant types you will have in your tank
  • The ambient location lighting

A lot will depend on the type of aquarium you have, and the type of fish you are keeping. Here are the average lighting requirements an aquarium should have daily.

  • An average aquarium – around ten hours per day
  • Tropical aquariums with plants – around 12 hours per day
  • Cold-water plants and cold-water fish – around 8 hours light per day

If you have a tank that isn’t using live plants, then you can get away with less light. However, some fish may not be appreciative of this amount of reduced lighting hours.

Once you look at these lighting figures, you can see half of the day needs the tank to be in darkness. This can affect the location where you can situate it.

The best locale is an area that is away from the most significant foot traffic, although still in an area where you can see it.

It is here many tank owners forget to turn off their lights to let their fish rest. These can be easily rectified with simple aquarium digital timers. You can set these to come on and go off as required. This enables any tank owner to replicate the natural habitats of their fish.

Aquarium Lighting Considerations

When you look at lighting for your aquarium, be sure to take into consideration the following.

  1. Are you providing lighting for a reef tank?
  2. Are you buying lighting for a planted aquarium?
  3. Do you need or want a timer to control the lighting automatically?
  4. Do you need an assembled light fixture or a retrofit light kit?

When you answer these questions, you soon see the direction you will be heading for your lighting, and thus the amount of darkness you will be delivering to your aquarium. These tips will help you along the way with your purchasing choice.

  • Fish-only tanks only need simple light systems: If there are no live plants to consider. You only have to deliver the right amount of light for your fish and to cover the deepest and widest areas of your tank.
  • When you have aquarium plants, you will need artificial light or natural light that is bright enough to allow the plants to perform photosynthesis.
  • If you have reef tanks, then you will need to think about the requirements for coral. These reef tanks may end up very different from a regular aquarium in lighting, and other areas of care. You may have to try multiple lighting setups before you find the perfect combination.


Once your lighting is chosen for your aquarium, you can control the light and darkness exposure for your fish by simply turning the light off every night. For tropical fish and plants, turning the light off with a timer means you don’t have to remember. If you have a cold-water aquarium, you may need to adjust the timer at specific times to copy how the seasons fall.