How Long to Leave Lights on in a Planted Aquarium

How Long to Leave Lights on in a Planted Aquarium

Knowing the amount of light needed for your planted aquarium is vital in ensuring its success. Light is, in fact, the most important aspect in the growth of any aquatic, or other aquarium plants. Without light, your plants will not grow. 

Deciding how much light is needed for your planted aquarium depends on a couple of factors. These include the species of plants you are planning to grow in your aquarium, how fast you want their growth, as well as the levels of carbon dioxide in your aquarium among others.

It is based on these factors that you’ll get to establish how long to leave your aquarium lights on.

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Lighting needs for aquarium plants

Different aquarium plants have differing light demands for their survival; some have higher demands than others. Often, the higher the lighting demands, the more difficult it is to grow the plant.

Glossostigma Elatinodes, for instance, has very high light demands. It requires extremely high light intensities for it to form a luxurious green carpet, and is otherwise quite challenging to grow. 

In addition, higher light demands often also call for high maintenance practices such as increased water changes, pruning, fertilization, and carbon dioxide demands.

This is because aquatic plants grow faster under high light intensities, which increases their absorption of carbon dioxide and nutrients. Not providing these two components will only lead to poor growth and algae filling your aquarium.

It is, however, easy to find yourself exaggerating the lighting levels in your aquarium. Note that too much light causes nuisance algae, and this a major challenge experienced by most aquarium hobbyists.

An easy way of reducing the lighting intensity in your aquarium is to position your lighting high above the water surface. You could also cover up or disconnect one of the bulbs.

Beginners are often advised to settle for planted aquariums with low lighting demands. Even though they grow slower, such plants are a lot easier to grow. The lower lighting demands mean that the plants require fewer amounts of carbon dioxide and nutrients to grow. In addition, the chances of algae outbreaks are also low.

Types of aquarium lighting

The most commonly used types of aquarium lighting are T5 and T8 fluorescent bulbs. Both are suitable options for growing plants but T5s are mostly recommended.

T5 fluorescent bulbs are more powerful than T8 bulbs. This makes them more suitable for the growth of aquarium plants in densely planted aquatic setups. A single full-length is enough for most planted aquariums. However, plants with high lighting demands might need two T5 bulbs.

Another type of aquarium lighting is LED lights. Hobbyists like them because their running cost is low and they offer incredible lighting effects. What’s more, a single LED light is capable of lasting for more than five years thus making them a good investment choice for your planted aquarium.

Lighting durations for planted aquariums

Whereas lighting in fish tanks is mainly for your viewing delight, aquatic plants rely on proper lighting for their survival. As mentioned earlier, without light, your aquarium plants will not grow.

Nonetheless, the light you choose to use in tour planted aquarium as well as the duration it stays on totally depends on the plant species in your tank. Some plant species do well in low lights and others need high lighting for longer. 

Low light aquatic plant species such as java fern, java moss, and anubias do well with approximately 8 to 10 hours of low light intensities per day. Experts recommend such plants for beginners and amateurs.

On the other hand, we also have light-hungry species. These aquatic plants need longer durations of lighting, somewhere in the region of about 12 hours per day. Such plants also demand higher amounts of fertilizer dosing and carbon dioxide for optimum growth.

That said, it is advisable to match your aquarium plants to the kind of fish that you have stocked in your tank; tropical fish with tropical plants and temperate fish with temperate plants.

Tropical fish such as the cory catfish, betta, and guppies should be stocked in planted aquariums with plants like cryptocoryne, hornwort, java moss, and java fern. Zebra danios, minnows, ricefish, and goldfish are temperate fish and should be stocked with aquatic plants like tiger lotus, and anubias.

Algae growth and light

The most conspicuous outcome of leaving your aquarium lights on for extended durations is algae. If you leave your aquarium lights running for more than 12 hours per day, it will only be a matter of time before you start noticing blue-green algae in your aquarium.

Note that it is not only your fish tank lighting that you should be worried about but also the natural light coming from the sun. Therefore, if your aquarium is housed in a brightly lit room, it is not advisable to place it too close to the room’s window. 

Unfortunately, algae will still probably grow in your tank regardless of whether or not there is too much. That said, what is of most importance is to keep in check the extent of the growth. In the case of algae is thriving in your aquarium even in somewhat low light, you should consider introducing algae-eaters such as snails and Amano shrimp into your tank.

Should aquarium lights be left on at night?

There is no doubt that proper lighting is important to your aquarium plants and fish, but so is the duration of darkness needed for them to replenish and rest. It is, therefore, best to switch off your aquarium lights at night in order to simulate the natural light cycle.

Did you know that fish actually do sleep! Most species, however, do not have any eyelids and this makes them rely on you to provide adequate hours of total darkness by turning off the lights for them to get some sleep. Simply put, leave your aquarium lights running for the recommended duration of 8 to 12 hours and then switch them off to simulate the natural day and night cycle.

If switching off the lights makes your fish restless, you could begin by switching off the room’s overhead lights first; about an hour before switching off the aquarium lights. This gives your fish time to at least adjust to the low lighting before being submerged into utter darkness. 

Moreover, it is not just fish that need time to rest and replenish; aquatic plants also need the same.

Lighting your aquarium while away from home

Perhaps you are out on a family vacation or have taken a road trip with family and friends. What should you with you regard to lighting your aquarium in such circumstances?

It is funny how most individuals will remember to leave food for their fish when spending some time away from home, but only a handful will remember scheduling their fish tank lights for when they aren’t around. Some people are not even sure whether or not they should leave the aquarium lights on while they are away.

An important thing to note is that your aquarium fish do not need constant lighting in order to survive.  Therefore. Leaving the lights switched off is totally okay. After all, leaving the lights running will encourage the growth of algae and can be a fire hazard.

Things are, however, different for planted aquariums. In such cases, you have to leave the lights switched on with a timer for the diurnal setting. On the other hand, if the room housing your aquarium is well lit, you could make use of the natural day and night cycle by leaving the aquarium close to a window.

Heat discharging lights and your aquarium’s temperature

This is something that is often overlooked by many. Apart from visible light, aquarium lighting fixtures also emanate and that is something that you need to take into consideration when choosing your aquarium’s light bulbs and fixtures. This is particularly important for the smaller tanks whereby there could be considerable increments in temperature. 

In general, incandescent bulbs and metal halides discharge large amounts of heat and are ideally not suited for aquariums. After all, incandescent bulbs are pretty much obsolete.

The more appropriate aquarium lighting source would thus be either fluorescent or LED. And between the two, LED is a better option. Consider the use of 

full spectrum LED rated lights (with a night mode) for planted aquariums.

One thing to note, however, if you decide to use fluorescent lights is that T8 or T5 fluorescent lights are not similar to VHO-fluorescent, which just like metal halides, increase the water temperature in your aquarium. In addition, T5 fluorescent bulbs are preferred over T12 and T8 because of their lower heat output and high efficiency.


Every aquarium surrounding is different and hence the only sure way of determining how long to leave your lights on is experimenting with different settings until you get the perfect fit for your aquarium.

Even so, you should take into consideration certain variables such as ambient and natural lighting. Always think of your plants before your fish; lighting is not such an important element for the survival of fish.

In addition, always conduct a bit of due diligence before replacing or purchasing aquarium lights. Compare the different products available to ensure that you are getting the best. And when deciding which lighting to purchase for your planted aquarium, always consider getting a programmable lighting fixture that has a timer, or one with night mode light settings.


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