How Long Should a Fish Tank Light Be On?

Becoming a new fish owner is exciting, but sometimes it can be overwhelming. Among the many questions that you’re likely asking yourself is—how long should a fish tank light be on?

You should keep a fish tank light on for an average of 10 to 12 hours. If you’re a family with young children like some of us, it’s a perfect opportunity to have your kids put your fish to “bed” before they also hit the hay.

Understanding Light’s Role in an Aquarium

The amount of time you should leave your fish tank light on depends on several factors, including your fish species and their preferred natural habitat.

But since most fish owners have several different fish species in their tank, plus the occasional shrimp and snail, it can feel challenging to strike the right light balance among them. 

For this reason, it’s safe to assume that 10 to 12 hours is an appropriate period, ensuring you provide areas of dense vegetation for fish that prefer less light.

When in doubt, sticking to a 12-hour light on and off schedule will help your fish maintain their circadian rhythm. Like humans, circadian rhythm allows fish to have a healthy sleep cycle, with light being one of the cues for them to know when they should be awake and when they should sleep.

Fish Light Options

A part of exploring the question, “How long should a fish tank light be on?” is to know the types of lights available for your aquarium. They are as follows:

  • Fluorescent
  • T-5 HO Fluorescent
  • LED Lighting

Of these options, LED is the most popular type of light to use in fish tanks. LED lights are an energy-efficient option that doesn’t overheat water tanks and maintains a consistent color.

In contrast, fluorescent lights are inexpensive, but you’ll spend money paying for a higher electricity bill. They also tend to shift in the color spectrum as they age. T-5 HO fluorescent lights are ideal for deep tanks, given that they give off a brighter light, allowing for deeper penetration.

How Long Should a Fish Tank Light Be On

How Long Should a Fish Tank Light Be On – Factors Impacting Tank Light Needs

Now that you know the basic answer to “How long should a fish tank light be on?” below are the most important factors when deciding whether to keep your fish tank light on the higher or lower end of the 10 to 12-hour recommendation.

Fish Species

Some fish enjoy living near the water’s surface and in crystal-clear water. Other fish are native to deep water habitats or shallower areas with dense plant vegetation, blocking most of the sun. And other fish are nocturnal.

So, knowing the preferred light requirements of your fish species in the wild will help you determine how long and how strong to set your tank light.

Plant Vegetation

Live plant vegetation has two sides when talking about light: It protects darker-loving fish from strong light, and it needs light to grow. Luckily most live plants thrive in aquariums with 10 to 12 hours of light.

However, certain plants, such as java moss and African water ferns, can live with as little as six hours of light per day.

Algae Growth

It’s common for beginners to struggle with algae growth as they learn the ropes of fishkeeping. It just so happens that one of the biggest triggers for fast algae growth is lots of light.

Therefore, if you’re constantly cleaning algae out of your tank, consider reducing the amount of daily light in your tank, as long as it’s still within an acceptable range for your fish. You can also put some algae-eating shrimp, snails, and fish in your tank to help control the problem.

Accounting for Invertebrates

Invertebrates like snails, shrimp, and crayfish typically enjoy dimmer tank settings. So, if you have light-loving fish in your tank in addition to these invertebrates, it’s vital to shelter them from the light.

Rocks, substrate they can burrow under, and areas with dense plants are all excellent solutions for keeping invertebrates safe in a well-lit tank.

Amount of Natural Light

Natural light plays a role in how much light your fish need. But it might not be how you expect; relying fully or partially on natural sunlight can negatively affect your fish.

That’s because your tank is a small space, meaning the water can heat up faster than you might expect. Direct sunlight can also spark algae blooms and increase your fish’s stress, potentially leading to disease. For this reason, you should keep your aquarium away from direct sunlight and rely on a tank light instead.


On average, most fish need light 10-12 hours a day. This range depends on the type of vegetation and species, such as invertebrates, that you have in your aquarium.

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