How Long Should The Light In A Fish Tank Be On? And How Others Do It

It was the first mistake I made. Leaving the lights on for too long. Thankfully I realized it in time before it got a problem for me.

So, how long should the light in a fish tank be on? 10 hours a day, but there are a lot of factors that make it vary between 8-12 hours as which type of plats and fish you have.

What the factors are, why these hours and how are others doing? It’s these questions we are going to take a deeper look at in this article.

Why These Hours?

Ten hours is what is recommended for most fish keepers since it will cover a wide range of different plants and species that needs a certain amount of light. Too much light and you might get a problem with algae growth, too little you might have a problem with your plants dying.

These numbers are with the light you have in your fish tank, not from external light sources such as sunlight. You should never have sunlight shining on your fish tank, neither direct or indirect. Especially if your room where you keep your fish tank has the sun shining the whole day.

The problem with the sunlight is that it’s more intense (the sun is bright you know) and the time you expose the aquarium for light will be longer.

What you should be using to light your fish tank is light sources above the fish tank. There are different kinds of light sources you could have with different properties. Those are incandescent lamps, fluorescent lamps and LED lamps.

The incandescent lamps are producing heat while the other two won’t do it. The problem with produced heat is that the fishes and plants to be damaged and you will also get more evaporation. Due to this heat-generating, it could be a good idea to lower the light hours closer to 8 hours a day.

Choose a good lamp, no matter what type of lamp you are using. Light is one of the most important things regarding plant life in a fish tank. If you are going with the stock light coming with your fish tank you are probably fine. Remember though that you can’t compensate bad lightning with longer light time! It will only cause problems with algae.

Plants And Light

The plants on land and in water are using the same mechanism to be able to live and grow. That mechanism is known as photosynthesis.

One input in the “photosynthesis equation” is light. More light makes more energy to the plant as long as the carbon dioxide is sufficient. Then you might be thinking “why not have the light on all the time?”.

It’s not that easy! Plants have a day and night cycle as we have and most plants are not able to photosynthesize more than 14 hours. There are also differences between the plant species of how much light they are able to use. Why not put it at 14 hours then?

14 hours of light could be okay for the plants but in the book “Ecology of the Planted Aquarium: A Practical Manual and Scientific Treatise for the Home Aquarist” the author Diana are stating that if you are giving the plants less than 12 hours of light a day, it could signal to the plants to stop growing.

In the book, she is recommending the “siesta technique”. It is that you have the light on for 5 hours in the morning and then let the lamp be turned off for 4 hours. Then let the lamp be on for 5 more hours in the evening.

That siesta period of 4 hours is beneficial since during that period the CO2 is boosted in the tank. CO2 is needed for photosynthesis. The bacteria metabolism is increased too which makes the environment in the fish tank better and the plants could use the light in the most effective way.

Fish And Light

For fish, it is not as important as it is for plants with light hours. But fishes have a day and night cycle as the plants and we do. Therefore it’s important to let the fishes have a period of some hours where they sleep. As with plants, how long that period needs to be is different from each species.

A good rule of thumb is to try to mimic how they are living in their natural habitat. If a fish is living in cold water where the light differs from season to season it could be a good idea to change the light in the fish tank too.

Some fishes are sensitive to temperature changes. These fishes might not be the best idea (although possible) to have lamps producing heat making the temperature fluctuate during the day.

If you are following to 10 hours “rule” you will be fine from a fish perspective. But as always, you should be reading about your inhabitants before you buy them to be sure of how much light they need.


Algae is loving light! That’s because they are a plant utilizing photosynthesis as normal plants do.

If algae are growing it’s a good sign that you might have too much light in your fish tank. It could be the sunlight shining in or it might be that you need to lower the number of hours on your lamp.

Algae are starting to grow when there isn’t enough competition of the nutrients from other plants. Be aware that it could be a problem with nutrition or anything else that makes algae grow, not only too many light hours.

To stop the algae from growing you could test lowering the hours of light in the fish tank. You could also, for some type of algae, completely darken the aquarium with a garbage bag or something similar for a few days. Don’t let the lamp be turned on during these days.

That makes the algae die due to lack of light. Usually, your plants are okay after that treatment and you will be algae free.

Help To Manage The Time

The best way to keep count of the hours the fish tank is lit is to use some sort of automatic timer. There are cheap ones that you plug into the power outlet and then plug your light in the timer.

There are also more advanced ones with apps. There is really no need for complicated timers for something as simple as a fish tank timer but it’s an alternative if you want.

You might not even need a timer. If your working schedule makes it possible you could turn the light on and off manually.

I would not recommend the manual method though. Most people are out of their house working and going to school and that makes it a lot harder to keep track and manage the hours.

Another upside with a timer is that when you are away for a longer period of time the timer will automatically fix the light hours for you without asking the one looking after your fish doing it.

How Are Others Doing With Their Light?

I was curious about how other fish keepers did with their lights and I asked around how many hours they had their fish tank lights on during the day.

With the knowledge I have, I was not surprised that most did have their lights on for 10 hours. I also asked if they have had any problems with algae and almost everyone said yes but the problem disappeared when they lowered the number of hours.

Most also had the light on for 10 hours during the afternoon. Not a lot had split the 10 hours into two parts. I, myself follow the siesta technique with good results. Since starting with that technique I have never had any light-related problems with my fish tank.

Related Questions

Are LED lights bad for fish? The fish doesn’t care as much as plants do and certainly not the type of light. LED is good since it’s not generating heat.

Can a fish live without light? It can but it’s not optimal since food will be harder to get


I have been a fish keeper a big part of my life. During the years I have gathered and searched a lot of information about fish keeping and here, on the website, is where I share that information to help you with your fish keeping. I want to provide the information that I didn't get when I had a question about my fish tank.

Recent Posts