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If you’re new to keeping live plants in your aquarium, you may be wondering how much light they need.  Can you just leave the light on all the time?  Do plants need a “resting” period where it’s dark in the aquarium?  And do all aquarium plants need the same type of light?

How many hours of light do aquarium plants need per day?  Most tropical tanks with aquarium plants will need around 12 hours of light per day.  Coldwater tanks with live plants (like goldfish tanks) only need about eight hours of light per day.

How much light your tank needs will vary, of course, depending on several factors, including:

  • How you’ve chosen to set up your tank and where it’s located
  • What plants you’ve got in the tank

I take a look at those factors and more in this article, including using timers and the role of CO2 in your tank.

Where is Your Aquarium Located?

Where you place your tank and how it’s set up has an impact on how long you’ll need to leave your aquarium lights on.

Aquariums placed in locations exposed to the sun won’t need the aquarium lights on as long as aquariums don’t get exposed to a lot of sunlight, and the same is true for aquariums in rooms with bright electric lights that are on a lot.

If you’re not sure whether your tank gets enough light without turning on your aquarium lights, you can always do the algae check.

If your tank has lots of green algae growing happily (or would have if you didn’t have Oto Cats, snails, or other algae eaters keeping it at bay), then your tank is probably getting enough light without using your aquarium lights, but it really won’t hurt to turn them on now and then.

aquarium with fish with glowing yellow light lamp on top in dark room.

Each Plant Has Unique Lighting Needs

The amount and type of lighting you’ll need in your aquarium also depends on the plant species and even the plant color!  Reddish plants will require more light and iron than green plants.  In addition, plants that like to live near the water’s surface in the wild will also need more light than plants that are happy along the bottom of the tank.

The chart below lists several common aquarium plants and their average light needs.

Aquarium Plant Light Needs
AnubiasLow light
Java FernLow to Medium light
Vallisneria (Jungle Val)Medium to High light
Java MossLow to High light
Pygmy Chain SwordMedium to High light
Amazon Sword PlantMedium to High light
CryptocoryneLow light
SagittariaHigh light
BambooHigh light
PogostemonHigh light
Rotala willichiiMedium to High light

Determining what “low,” “medium,” and “high” light means is a little tricky, and goes by watts of light per gallon of water, or wpg.  Normally, low light is around 1.5 watts per gallon, medium-light is 2 watts per gallon, and high light is 3 watts per gallon.

If you’re a beginner in this hobby, this information may not make sense to you yet. The best thing you can do is check if your plants are reaching or starting to brown, which means your light may be too weak for your plants.

On the other hand, if you have low-light plants and a lot of bright light, you might need to tone it down a bit or your tank could be overtaken by algae.

The good news is that most plants will still be healthy in any level of light, but you might notice that:

  • With lower levels of light, some plants grow slowly or not at all
  • With higher levels of light, some plants take off and may overwhelm your tank, requiring a lot of pruning to stay under control

If you’re not happy with how your plants look, experiment with how much light they get.  Leave the light on for a longer period of time, or change to alight with a red light spectrum or one labeled for plant growth.

Also, don’t forget to check the type of plant you have against the chart above to see if you’re hitting your plant’s light needs.

 A Word About CO2

Just like outdoor plants, your aquarium plants take in carbon dioxide and produce oxygen as part of photosynthesis.  They take the 2 oxygen that is part of carbon dioxide and releases them back out while keeping the carbon for themselves.

There are a lot of sites out there telling aquarium owners they need to add carbon dioxide to their planted tanks, but is that true? To put it simply:

CO2 is recommended for tanks with strong lighting and high-light demanding plants. 

CO2 isn’t necessary for aquariums that have low light plants and weak lighting. 

For most fish tanks, there’s no need to add extra carbon dioxide in the tank since there’s a small amount already there.  Similar to us humans, your fish take oxygen from the water and give off carbon dioxide.  Normally, there’s already enough dissolved carbon dioxide in the water from the fish for the plants to use without needing anything extra.

There are many aquarium plants that float on the surface of the tank or break the water’s surface, and they can easily get additional carbon dioxide from the air.

However, if you have a very heavily planted tank, it’s possible there might not be enough fish in the tank to provide all the carbon dioxide your plants will need.

Another reason you may need to add carbon dioxide is if your tank is consistently lit at more than 3 watts per gallon, which is a high amount of light, and you don’t want to have massive algae blooms.  In that case, you’ll probably need to add some carbon dioxide to your tank to keep the algae in check.

If you do choose to add CO2 to the tank, be careful not to overdo it.  Too much carbon dioxide will suffocate your fish because there will be too much carbon dioxide and not enough oxygen.

Adding carbon dioxide when the tank is dark could suffocate your fish as well since plants don’t use carbon dioxide up when it’s dark because photosynthesis requires light to work.

Using Timers To Regulate Lights and CO2

I’ve used timers in the past for my aquarium lights, and one thing I loved about using them is knowing that the lights will be on for exactly the same amount of time each day.

This is great for the fishkeeper on the go, especially if you’re not always able to be home when you want the lights to come on in the aquarium.

There are hour manual timers, digital timers, timers that can be set for a certain number of hours, and you can even set weekly timer schedules.

Most are pretty easy to set up—you just plug your light into the timer and then set it according to the instructions for whatever amount of time you’d like.

The only thing to remember about timers is they can’t adjust for time changes or less light in the winter without your help.

Measuring “Light” In An Aquarium

It’s difficult to know how much light your aquarium is getting without an No products found. if you don’t know how your aquarium light is rated, or what the ratings mean. Did you know that there are four types of measurements for aquarium “light”?

They are:

  • Wattage (how much energy the light uses, or power)
  • Kelvins (the spectrum, or color wavelength of the light)
  • Lumens (brightness)
  • PAR (“Photosynthetically Active Radiation,” or in simple terms, how much of the light is useful for plants)

So which type of light grows plants the best?  Studies have shown that plants grow best with aquarium lighting that has more red or green in the color spectrum than blue, and the redder the better!

The image below shows a red-spectrum fluorescent light used for growing aquarium plants.  Notice how much more red light it puts out than the full-spectrum daylight lamp above.

In Summary

Live plants add a lot to an aquarium, and having the right lighting to keep them healthy and happy is essential.

Make sure to keep the lights on long enough for the type of plant, location of the tank, and the type of tank set-up you have, and your plants will reward you with plenty of growth.  For most tropical aquariums, this means getting about 8-12 hours of light per day.  To make it even easier, you can always set up a timer to ensure your lights stay on for the same amount of time every day.

If you really want your plants to take off, try to get a light that puts off more light in the red wavelength.

If you like the article above, here are some other similar articles you should check out!

What’s the Difference between Emersed and Submerged Aquarium Plants?

Where does Aquarium Driftwood come from?

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Hi, my name is Jordan. I've been in the fishkeeping hobby since my childhood. Welcome to my blog where I help fishkeepers enjoy the hobby by offering free guides, advice, & product reviews. Read more...