How Long Should Aquarium Lights Be On? Light Schedules & More

    how long should aquarium lights be on?

    A big part of fishkeeping revolves around mirroring the natural habitat of fish inside the aquarium. Figuring out a healthy lighting schedule is a key part of that process! How long should aquarium lights be on? Can fish live without lights in a tank? How does light influence algae & plant growth?

    Aquarium fish, and live plants, need between 8-12 hours of exposure to light per day. The ideal light schedule will depend on the species you’re keeping. The goal is to have fish thriving, plants growing, and algae under control!

    Having the lights on 24h/day in a fish tank or providing no supplemental light can both be damaging in different ways. Recreating the water conditions that your fish are native to will help keep them stress-free and healthy.  

    How Long Should Aquarium Lights Be On? It Depends.

    How long should aquarium lights be on

    The ideal light schedule for a fish tank depends on a few interconnected factors:

    How many fish you’re keeping in an aquarium (stocking capacity)

    A fish tank stocked to 85% of its capacity (such as a community tank) will usually be home to fish with different behavior patterns. Keeping the lights on for 12 hours and off for 12 hours helps the following natural events take place:

    • Fish establish their circadian rhythm, being active and feeding during daytime and resting when it’s dark. This pattern in activity level and metabolism helps keep fish healthy and extends their lifespans. 
    • Nocturnal fish (soldierfish, fire eels) and invertebrates will follow their natural instincts at night. When the lights are off, nocturnal species will confidently explore the tank and scavenge for food. 

    In a less stocked single-species tank, you can use a light schedule that’s suitable for that species alone.

    What types of fish you’re keeping

    Tropical fish, coldwater fish, marine fish, and invertebrates each have slightly different light needs when in captivity. None are polar opposites when it comes to a light schedule, but they do have distinct notable preferences.

    These preferences, for more or less exposure to light, are directly connected to the bodies of water that the species are native to.

    Aquarium fish indigenous to rivers and crystal-clear lakes will need 10-12 hours of bright light to thrive. On the other hand, coldwater fish (white cloud mountain minnows, archer fish) and some invertebrates prefer dimmer lights for about 8 hours/day.

    How heavily planted the tank is

    Live aquarium plants have 3 basic needs: sufficient light, adequate temperature, and nutrients (+ CO2 for some). Like fish, different plant species will have similar but distinct needs when it comes to light.

    Tropical plants with fast growth rates will need 12 hours of exposure to bright high-intensity lighting. Similarly, a heavily planted tank will require high lights for 10-12 hours to keep the plants healthy. Conveniently enough, some of the most popular low-light plants are also beginner-friendly.

    Anubias Nana, Java Moss, and African Water Ferns are all considered low-maintenance plants. And that’s because they can grow with even just 6-8 hours of light per day.

    What other critters are inhabiting the tank

    A shrimp tank, or an aquarium dedicated to invertebrates, will need significantly less light than a tropical fish tank. Most invertebrates (shrimp, snails, crayfish, etc.) will actually seek coverage when kept in high light community tanks.

    What happens when invertebrates share a tank with fish or live plants that need 10-12 hours of light per day? You need to make sure there’s a thick layer of substrate for them to burrow into. Hiding spots and decorations that invertebrates can use to stay in the shade are also highly recommended.

    Whether or not algae are a nuisance or a necessity in your fish tank

    Some fishkeepers use algae’s accelerated growth rate in the presence of strong light to their advantage. Algae are necessary for a self-sustaining tank full of algae-eaters (Bristlenose plecos, Siamese algae eaters, shrimp, snails, etc.). In this instance, more light = more food with minimal human intervention.

    On the other hand, some aquarists are on a constant crusade against algae overgrowth. The first step a fishkeeper can take to get on top of algae control is to cut back on light exposure.

    Algae blooms are closely connected to a fish tank getting too much light. This can happen when aquariums are lit 24/7 or if a tank’s location exposes it to too much direct sunlight.

    How much light the aquarium is exposed to by default

    Natural ambient light can influence what your fish tank’s light schedule should look like. Placing a fish tank near windows that get plenty of direct sunlight isn’t ideal for stable water conditions. But with constant exposure to sun rays, you can keep aquarium lights off longer.

    Relying on ambient lighting isn’t typically recommended. Aquarium plants rarely get enough light this way, which prevents them from growing and developing strong root systems.

    Relying on sunlight to meet your fish tank’s light requirements can trigger some pretty dangerous side effects:

    • Algae blooms;
    • Unstable water temperature, with major fluctuations;
    • Increased stress levels in fish, which leaves them vulnerable to diseases.  

    How Long Should Lights Be On In A Freshwater Aquarium?

    best time to turn on aquarium light

    Here are a few light schedule guidelines for aquarium fish (& critters) based on what species you’re keeping:

    Tropical fish

    Most freshwater tropical fish do best with a 12 hours on / 12 hours off light schedule. This mimics the water conditions of the habitats that they are native to.

    Unless you’re keeping tropical fish in a heavily planted tank with tropical plants, most aquarium light fixtures will do the job just fine.

    As with every rule, there are exceptions. Cichlids that originate from the bottoms of dimly lit bodies of water will require less light. Keeping the lights on for 8-9 hours per day will work better in an African cichlid tank.

    Coldwater fish

    Coldwater fish are adapted to living in environments with cooler water temperatures and less light. 

    In the wild, fish species with low light needs usually live in areas with fewer daylight hours, depending on the season.

    You’ll still need to keep the lights on for about 8 hours/day in an aquarium stocked with coldwater fish. 

    If you’re setting up a planted tank, make sure you include live plants with similar light needs. 

    How Long Should Lights Be On In A Freshwater Aquarium?

    Tropical Fish – Lights on for 12 hours/day

    Coldwater Fish – Lights on for 8 hours/day

    Bettas Goldfish
    Angelfish Koi Fish
    Guppy Fish White Cloud Mountain Minnows
    Rainbowfish Zebrafish
    Tetras Hillstream Loaches

    How Long Should Lights Be On In A Saltwater Aquarium?

    how long should aquarium lights be on for plants

    Marine aquarium fish will typically need the lights to be on between 8-10 hours per day. 

    This diurnal light schedule is similar to the one they would follow if they were living in the wild.

    Reef tanks, with growing corals and sea anemones, will require special lighting, and a daytime length of 10-12 hours. For good coral growth and color development, fishkeepers usually use cool white to blue light.

    Check out this reef aquarium lighting guide for a much more detailed take on coral light requirements:

    How Long Should Lights Be On In A Saltwater Aquarium?

    Saltwater Aquarium – Lights on for 8-10 hours/day

    Reef tank – Lights on for 10-12 hours/day

    Pufferfish Yellow Tangs
    Maroon Clownfish Blue Reef Chromis
    Marine Wrasses Neon Blue Goby
    Filefish Firefish
    Snowflake Moray Mandarinfish


    Do Aquarium Fish Need Light?

    Aquarium fish require light for various biological and behavioral reasons.

    Light plays a crucial role in the fish’s natural habitat, providing them with the necessary energy for growth and reproduction.

    In an aquarium setting, artificial lighting is indispensable as it helps simulate daylight conditions and stimulates the fish’s internal clock.

    It also promotes the production of vitamin D, which aids in the absorption of calcium and strengthens their bones and scales.

    Maintaining a proper light cycle is essential to replicate their natural environment and prevent disruptions in their physiological processes. 

    Invertebrate Light Schedule

    how many hours of light in a fish tank

    Freshwater invertebrates (shrimp, snails, crayfish, crabs) don’t necessarily depend on light to thrive. But light plays an essential role in the presence of their favorite food source: algae!

    You’ll rarely see invertebrates exploring the tank when strong aquarium lights are on. They tend to use live plants and décor items as hiding spots, waiting for nighttime to forage for food.

    No light at all wouldn’t be beneficial to them, as there would be fewer algae to munch on. But, keeping the lights off for 13-14 hours will encourage these algae-eaters to feed and clean your tank in the process!

    How Does Not Enough Or Too Much Light Affect Fish?

    How long should aquarium lights be on

    An adequate light schedule helps fish adjust to a diurnal circadian rhythm, which is essential to good health and a strong immune system. With 8 to 12 hours of light per day, fish adapt to swimming, exploring, and feeding during the daytime.

    Resting while the lights are off helps to keep stress levels low, which is the most crucial factor in keeping aquarium fish healthy. While having their light requirements met, fish also display more vibrant colorations!

    So, how can you tell if fish aren’t getting enough light?

    Here’s a brief list of possible signs that you need either higher-intensity lights or more hours of daylight:

    • Slow growth rates, compared to the average growth rate of the species you’re keeping;
    • Lethargic movement of otherwise active fish;
    • Discoloration or faded colors in fish, live plants, or corals;
    • Lack of appetite;
    • Abnormal behavior that suggests your pet fish are in distress.

    If your fish are getting too much light, you’ll see surface-feeders and mid-to-top level swimmers hide near the bottom of the tankmid-to-top level swimmers hide near the bottom of the tank.

    This change in behavior can indicate that your fish don’t need as much light as you think they do. 

    You can start by turning the lights off one hour earlier daily to see if that makes a difference.

    How Many Hours Of Light Do Aquarium Plants Need Per Day?

    Live aquarium plants need exposure to light for up to 12 hours/day. Like fish, different species of plants will have varying light requirements. But for optimal growth, live plants will need a mix of ambient lighting and artificial tank lights. 

    Aquarium plants need light to perform photosynthesis. Giving aquatic plants sufficient light is crucial when setting up a planted tank. This allows them to fulfill their role as water oxygenators and purifiers.

    By feeding on the byproducts of the nitrogen cycle, plants remove ammonia, nitrites, and nitrates, working to improve water quality continuously. In well-lit, heavily planted tanks, plants also help keep algae under control by outcompeting them for nutrients.

    All of the above considered, it makes sense that fishkeepers don’t shy away from investing resources to meet the tank requirements of live plants.

    Based on the origin of the aquarium plant species you’re keeping, they will need between 8-12 hours of light per day. Tropical fast-growing plants will require up to 12 daylight hours under stronger lights to develop to their full potential.

    On the opposite side of the spectrum, coldwater plants can flourish with just 8 hours of exposure to medium to high-intensity lights.

    Here’s a light schedule guide for some of the most popular aquarium plants in the hobby:

    How long should aquarium lights be on in a Planted Tank?

    Tropical plants – Lights on 12 hours/day

    Coldwater plants – Lights on 8 hours/day

    Red Leaf Ludwigia Salvinia natans
    Nymphaea Lotus Coontail
    Beckett’s Water Trumpet Anubias
    Bacopa caroliniana Java Fern
    Dwarf Baby Tears Marimo Moss
    Scarlet Temple Christmas Moss
    Monte Carlo Plant Java Moss
    Ammannia gracilis Waterweeds
    Staurogyne repens Duckweeds

    Is It Ok To Leave Aquarium Lights On All The Time?

    You shouldn’t leave aquarium lights on 24/7. Here’s why:

    • It can increase stress levels among your pet fish

    With no distinct cue to rest (lights getting turned off), fish in captivity can end up with a dysregulated circadian rhythm. This can negatively impact their metabolism, their behavior, and their health in general.

    Stress among fish in an aquarium can set off a cohort of other side effects: vulnerability to diseases, fungal infections, parasitic infestations, etc.

    • It can cause algae overgrowth. 

    Along with an excess of phosphorus and nitrogen, too much light, of strong intensity, can cause never-ending algae to bloom in an aquarium.

    Keep lights on for no more than 8 hours/day, and move the fish tank away from direct sunlight. 

    In most cases, these two steps can get your algae overgrowth problem under control.

    • It can trigger fluctuations in water parameters

    If your tank set-up includes incandescent/fluorescent lights, they can overheat a smaller aquarium. Especially if you were to keep the lights on day and night!

    Sudden fluctuations in water temperature can negatively impact water quality parameters, like causing oxygenation levels to drop.

    FAQs About How Long Should A Fish Tank Light Be On

    When Is The Best Time To Turn On Aquarium Light?

    The best time to turn on aquarium lights varies depending on the type of fish and plants in the tank. Generally, it is recommended to provide 8-12 hours of light per day, simulating a natural day-night cycle.

    It is best to turn on the aquarium lights in the morning and turn them off in the evening to ensure a healthy environment for your aquatic pets.

    Should I Leave My Fish Tank Light On During The Day?

    It is generally recommended to leave your fish tank light on for around 8-12 hours during the day.

    This helps mimic their natural environment and promotes their well-being.

    However, certain fish species may require different lighting conditions, so it’s important to research and understand the specific needs of your fish to provide them with optimal care.

    Should You Turn Aquarium Lights Off At Night?

    Yes! The best time to turn off aquarium lights is when the ambient lighting in the room also gets darker.

    This helps fish self-regulate, developing routines similar to our own.

    This also means you get to enjoy your fish being active and swimming during the daytime while you’re awake too. Fish metabolism also follows this diurnal pattern, so you can feed the fish during the day.

    Turning lights off at night is also a cue for nocturnal species to start feeding and exploring the aquarium!

    Can Aquarium Lights Be Off For 24 Hours?

    A 24-hour energy blackout or simply being away for a day in an emergency situation wouldn’t put your fish in danger. As long as fish get at least some ambient lighting, they should be just fine. Your live plants might be a bit dramatic and not look their best, but they too can survive.

    Are you planning on going on vacation, and you don’t want to burden the fish sitter with following a light schedule? Use a 24h digital timer to make sure your fish & plants get their light requirements met even when you’re away.

    Can Fish Survive In An Aquarium Without Lights?

    Although some fish have adapted to require no light at all, those fish live at impressive depths. 

    Aquarium fish, especially those bred and raised in captivity, need exposure to light to survive.

    Even coldwater fish that thrive on just 8 hours of light/day would be distressed having to rely on ambient lighting alone.

    Recreating the conditions that aquarium fish are evolutionarily adapted to is a responsibility we, as fishkeepers, need to fulfill. You can get affordable overhead LED clamp lights if you’re on a tight fishkeeping budget. 

    Can Fish Tank Lights Be Too Bright?

    Aquarium lights can be too bright if you’re using incandescent light bulbs. These generate heat and can cause significant water temperature fluctuations, especially in a nano tank (5-20 gallons).

    A quick way to tell if the lights are too bright for your fish is to monitor their behavior. All your fish suddenly preferring just certain areas of the tank is a big red flag. You’ll usually see them crowding shadier spots or staying near the filter’s output.

    Switching your light fixture to LED lights should solve the problem immediately. LEDs don’t generate heat, and they’re ideal even for heavily planted tanks.


    Having aquarium lights on for 10-12 hours is beneficial for most tank set-ups. The light requirements for heavily planted tanks versus a bare aquarium for coldwater fish vary by about 4 hours.

    You’ll see major improvements in fish behavior & health once you start catering to their particular light preferences. You can also stimulate plant growth using the same principle by adjusting the light schedule to the needs of the species you’re keeping.

    Keeping aquarium lights on at all times is a recipe for disaster (stressed-out fish and algae blooms galore!). Just like relying on ambient lighting isn’t a responsible approach either.

    Feel like your fishkeeping to-do list is getting too long? Make following a light schedule effortless by hooking your aquarium lights up to a digital day/night timer!

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