How Long Can Algae Live Without Light

How Long Can Algae Live Without Light?

Since algae are photosynthetic organisms, they need light to grow and survive. By blocking out the light to an aquarium tank for four to seven days, most algae will be completely eliminated.

Although algae need light to live, they are still much hardier than you would think. If you want to kill off the algal growth in your fish tank, you can deprive the tank of all light. However, it will take at least four days to accomplish your task and can even take up to two weeks for species to die off without light exposure.

In this article, we’ll dive into whether or not algae can grow in your fish tank without light, if they can grow under artificial light conditions, and if the solution to killing the algae in your aquarium is to turn off the lights.

Get ready to learn all about the light requirements of algae so that you can take appropriate action to eliminating an overgrowth of algae that doesn’t involve chemicals or loading up your tank with an absurd number of Plecos.

Can Algae Grow Without Light?

Like plants, most species of algae grow via photosynthesis. They convert sunlight and carbon dioxide into oxygen and energy which fuels their growth. If you take away one of these ingredients, e.g. light, then growth cannot take place.

Although light deprivation will not immediately kill algae, it will hinder their growth. The first thing that happens to algae when you cut off its light supply, is the termination of it getting any bigger. If you are looking to stop the algae from growing in your aquarium tank, consider a light deprivation treatment.

Move your tank into a windowless room or cover it with light blocking material. If you observe how this treatment affects the algae, you won’t begin to notice a difference until a day or two in. The most noticeable change to your fish tank is that the algae will not be doubling in appearance as it had previously.

After a week or two, the algae will die off, but the hindrance of growth is the first step. Take precautions with the other inhabitants of the tank, especially your aquatic vegetation. For as much as algae need light, so do your plants. A light deprivation treatment will kill your vegetation so make sure to remove it from the tank before killing off the algae.

Can Algae Grow With Artificial Light?

There is much debate over the role that artificial lighting plays in the growth of aquarium algae. There are those that vehemently deny that artificial light is to blame for the overgrowth of algae while other are 100% behind blaming artificial light. The reality lies somewhere in the middle.

If the artificial lighting has the proper wavelengths for photosynthesis, then it will definitely allow algae to grow. However, artificial lighting is not as effective at helping algae grow as direct sunlight. If you are worried about your aquarium lighting promoting algal growth, take a look at the type of lighting you have.

There are certain brands of aquarium lighting that are specifically constructed to give your fish the light they need without promoting the growth of algae. Just remember that if you need light for your plants to photosynthesize, whether it is artificial or not, then the algae will also benefit from light exposure.

Whether artificial lights allow algae to grow or not, you are better off placing your fish tank under artificial lights rather than in a sunlit window to discourage algal growth.

Will Turning Lights Off Kill Algae?

If you want to kill the algae in your fish tank, simply turning off the lights will not do the trick. Unless your fish tank lives in a room completely walled off from the sun and blocked off from the rest of the house so that no one ever enters, it will still receive light even if the lights are off.

Managing the amount of time per day that your aquarium lights are on will help decrease the likelihood of algae overtaking your tank. Your fish and aquatic plants do not need light all the time, so having an appropriate light cycle will actually benefit all of the inhabitants of your tank.

Instead of turning off the lights to kill the algae in your tank, try covering the tank with a light impenetrable material. This will be more effective at blocking out the type of light algae needs to photosynthesize, grow, and reproduce.

Move your aquarium away from any windows to avoid excess sunlight from fueling the growth of algae. Even if you turn off all the lights in the room including the aquarium light, the sun can provide more than enough light that algae need to grow.


If your aquarium tank is overrun by algae, there is a simple chemical-free solution to your problem. Algae need light in order to grow and thrive. If you deprive the algae in your fish tank of light, they will eventually die. However, algae are very resilient, and the treatment could take as long as two weeks in order to be successful.

Cutting off the algae’s light supply will first inhibit growth and then eventually kill it. Some artificial lights can serve as proxies for sunlight and may help algae grow. To be on the safe side, keep your lights on a manageable schedule so that you can cut down on algal growth and encourage an appropriate circadian rhythm for your tank inhabitants.

The best light for algal growth though is sunlight. Natural sunlight is exactly what growing algae need so make sure to eliminate your aquarium tank’s exposure as much as possible. Avoid placing your tank in a window or a place that receives a lot of sunlight. You do not need to put it in a windowless room, but it is advisable to use curtains to block direct sunlight.

Go cover up your tank and watch the algae disappear!