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Is It Bad to Leave The Fish Tank Light On? (Important to know)

Aquarium lighting is essential to the health of both fish and plants in a tank. It plays an important role not only in mimicking the natural environment of the fish but also bringing the tank to life! Right now, you may be wondering to yourself, ‘is it okay (or not) to leave the tank light on all the time?’

Is it bad to leave fish tank light on? The answer is, yes, if you leave it on all the time. Tank lighting should be left on for no more than 12 hours per day. All fish require periods of darkness, similar to their natural aquatic environments. Leaving your tank light on 24/7 will result in the overgrowth of algae and possibly harm the fish.

Now that you know you shouldn’t leave your fish tank light on all the time, let’s take a closer look at this in more detail below. We’ll discuss how long you should leave the light on and whether or not LED lights are good for fish. We’ll also explore how light affects fish and if they can ‘sleep’ in blue light.

So, if you’re ready to learn more about tank lighting and how it affects fish, then let’s get started!


How Long Should You Leave the Light on in a Fish Tank?

Aquarium tank lighting should be left on for 12 hours at the most, if you have tropical fish and plants. To recreate tropical conditions, there must be equal hours of light and darkness each day. Coldwater fish and plants, on the other hand, live in aquatic areas with fluctuating daylight hours. Therefore, it is best to match the seasonal daylight hours with your aquarium lighting. Saltwater fish, depending on the type of aquarium you have, need 8 to 10 hours of light, to best simulate their natural aquatic surroundings.


Should I Turn My Fish Aquarium Light Off at Night?

As most fish require both periods of light and darkness each day, turning your aquarium light off at night is a good idea. It will mimic the natural cycle of daylight and provide the fish and plants with ample light to be healthy and vibrant. For optimal visual enjoyment, it is recommended that you leave the tank light on during the day (so you can clearly view the fish) and then turn if off when you go to sleep at night.


Can Fish Sleep in Blue Light?

A blue ambient light in an aquarium is meant to mimic moonlight. They are a great addition to any tank as they don’t affect the day and night cycle of the fish or plants and don’t contribute to the growth of algae. In it, nocturnal fish are free to swim about or ‘sleep’ (with their eyes open, as they don’t have eyelids), which is more like a resting period than an actual slumber.


Can Fish Live Without Light?

Many saltwater fish can live without light, especially those who dwell at depths greater than 1000 metres. However, most aquarium fish require some light during the day as they haven’t evolved enough to live in completely dark surroundings. A minimum of 8 hours of light per day is recommended, with a maximum of 12 hours. Fish, like plants, have a biological ‘need’ for sunlight, therefore, tank lighting is a must if you hope to maintain a healthy and thriving aquarium.


Do Fish Tanks Need Light?

All fish tanks need light, as most fish require both periods of light and darkness throughout the day. Tank lighting not only helps regulate the sleeping and eating habits of your fish but it also enhances fish colors and makes it easier to spot any potential problems. Light provides energy and oxygen to fish while contributing to the health and growth of the plants and corals in the tank as well.

*** Interested in finding out how you can improve your aquarium fish’s coloring? If so check out my article on the best aquarium lighting for fish color ***

Are Lights Bad for Fish?

Lights are not bad for fish, unless they are too bright or cause fluctuations in aquarium water temperature. While fish have a biological ‘need’ for light and should have at least 8 hours of light per day, they are not as reliant on it as plants in a tank.

Aquatic plants need as much as 12 hours of light per day to grow and thrive in an aquarium. And since many fish rely on plants for food, oxygen and coverage, proper lighting in an aquarium is a must! Plants keep the water and fish healthy by absorbing carbon dioxide and ammonia while helping to recreate a natural ecosystem, similar to artificial lighting in a tank.


Are LED Lights Good for Fish?

LED lights in an aquarium are better than fluorescent or incandescent types, for example. Fluorescent and incandescent bulbs generate higher heat than LED lights, causing fluctuations in water temperature which can be dangerous to certain types of fish. If this is the case, you may see fish swimming around a smaller, confined area in the tank when the temperature is cooler. This is usually a sign of distress.

Although LED lights may be more expensive to purchase outright, they save money on power in the long run. Generally speaking, they are more cost-effective than fluorescent or incandescent lighting. They are also safer for fish as they promote the growth of live plants in a tank, which helps to reduce the overgrowth of algae.

*** I really like the Koval brand of lights. I like then so much I wrote a review on them. Please check it out if you are considering purchasing LED lighting for your fish tank ***


How Does Light Affect Fish?

Light has a profound effect of how fish function in an aquarium. It not only influences their developmental growth but also their metabolism. If they do not receive a sufficient amount of light throughout the day, they could suffer from stunted growth and/or lethargy. Proper lighting, such as that provided by LED systems, will showcase fish color better, especially shades of blue!

Fish are generally more alert and active in brighter light. Therefore, if you want to have a healthy, vibrant tank, then you need to provide your aquarium with sufficient overhead lighting. Be sure that the light isn’t too bright however, or your fish will react by hiding in amongst the plants and/or diving deep down near the substrate when the light is turned on. This is yet another reason why LED lights are better than fluorescent or incandescent types.



To conclude, it is not recommended that you leave your aquarium light on all the time. Tank lighting should be left on for approximately 8 to 10 hours and no more than 12 hours per day. All fish (and plants) require periods of darkness in a day, similar to life in their natural environments. Leaving your tank light on 24/7 will result in the overgrowth of algae and possibly increase the stress levels of the fish.

Happy fish keeping!


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