Planted Aquarium Lighting Guide – What Type, How Much and How Long?

When first starting out, most fishkeepers go for silk or plastic plants – if they have any at all – just to keep things simple.

This is a wise move in most cases, but there is a range of benefits to having live plants in your aquarium.

Not only do they look nice and create a more natural environment for the residents in your tank, they also produce oxygen and absorb ammonia and carbon dioxide, so they help keep the water in the tank healthy.

They’re also a good source of food for many species, and one that keeps replenishing itself.

Lighting for planted Aquariums

The main problem is that many people have trouble keeping live plants, well, live. It’s not that they’re especially hard to keep it’s just that you can’t expect to put them in an aquarium that’s not set up for housing live plants and expect them to survive.

One of the key factors in keeping your aquarium plants alive is having the correct lighting.

In this article, we’re going to cover the basics of lighting for freshwater planted aquariums.

This is a guide for newbies to the realm of planted aquariums. So, if you’re an expert looking for more detailed or specific advice, sorry; this isn’t the place for you.

But, if you’re just starting out, read on and we’ll give you a good primer to see you on your way.

Why Must We Light a Planted Tank?

If you paid attention in Biology class, you’ll already know the basics of why a planted tank has to be properly lighted.

In the most straightforward terms, plants need light to live.

While animals eat to get everything they need to grow and stay alive, plants do this using light (as well as water and carbon dioxide) via a process called photosynthesis.

Special photosynthetic cells containing chlorophyll trap light energy and use it to turn the carbon dioxide and water they’ve absorbed from the atmosphere around them into glucose, which plants need to stay alive.

Oxygen is released as a bi-product of this process (which is lucky for all of us, otherwise we wouldn’t be alive).

While, in nature, plants would trap energy from sunlight, when you’re keeping them in an aquarium indoors, the amount of natural light that comes into your house just isn’t enough for photosynthesis.

If you don’t have the right kind of lights for a planted tank, anything you try to grow will die.

Intensity of Light: Watts per Gallon and PAR

To figure out the amount of light you need to grow and maintain certain plants, you need to be able to measure it. Or, more specifically, the intensity of light.

When discussing aquarium plants, two main units exist for measuring light: photosynthetically active radiation (PAR) and watts per gallon.

Watts per Gallon

Watts per gallon used to be the standard unit for measuring the light intensity in an aquarium, probably because it’s so simple to figure out.

Assuming you know the wattage of the bulbs you’re using and the size of your tank in gallons, you just have to divide the amount of watts by the amount of gallons and you have your watts per gallon measurement.

Say you were running two 20 watt bulbs (a total of 40 watts) in your 20 gallon tank, that would be 2 watts per gallon (figured out by dividing 40 by 20).

Simple, yes, but a number of issues exist with this way of measuring light. The intensity of light will reduce the further away the bulb is from the substrate, for instance. So, it’s not the most accurate measurement.


PAR is a unit of measurement based on the way that plants “perceive” light. One PAR is one millionth of a mole of photons striking a one square meter area every second.

Sounds complicated, right? Well, it kind of is, which is why it hasn’t historically been used as a light intensity measurement for growing plants in home aquariums.

PAR can only be measured using a PAR meter and, until a handful of years ago, these cost thousands of dollars.

Now you can buy one for an average of a few hundred, which is much more reasonable, but still a big outlay for a beginner.

That’s why, even though it’s not as accurate, we’ll be using the measurement watts per gallon in this article, rather than PAR.

Watts per gallon is absolutely fine when you’re growing hardy beginner plants. However, if you want to progress to serious aquascaping, creating highly-planted aquariums and featuring esoteric plants that are more difficult to grow, you will need to invest in a PAR meter.

Spectrum of Light

If you haven’t ever thought about it much, you might think there’s only one type of light. That it’s sort of yellowish or whiteish and that’s the end of that.

But light can come in a range of color temperatures. Daylight is generally considered full spectrum light, so it contains all the color temperatures of the electromagnetic spectrum, from infrared to ultraviolet (or near-ultraviolet at least).

Color temperature is measured in Kelvins, and any aquarium lighting you buy should say where it falls on the Kelvin scale.

This is important, because plants predominantly need light that falls in both the red spectrum and the blue spectrum to photosynthesize.

Because the majority of freshwater aquatic plants commonly available today were originally brought over from shallow tributaries and rivers in Central and South America, they require a full light spectrum range similar to that of natural daylight. On the Kelvin scale, this falls somewhere between 5500 K and 7500 K.

Types of Lighting

Now that you know more about why planted tanks need lighting, how the intensity of it is measured, and how the spectrum of it is measured, you might like some more practical advice about lighting your freshwater planted tank.

Planted Aquarium Light

These are the main kinds of lighting available.

Fluorescent Tube Lighting

Fluorescent tube lights are among the most commonly used aquarium bulbs. They’re cheap to run and are relatively energy efficient, so they’re good all-rounders that won’t break the bank.

They’re available in different color ranges, so be sure to pick a full-spectrum variety for your planted aquarium.

Incandescent Bulbs

These used to be pretty standard in small aquariums, but with advances in energy-efficient bulbs, they’ve been all but discarded.

They use a lot of energy, heat up quite warm (and so might affect the temperature of the water in your tank), and don’t come in the full-spectrum necessary to maintain healthy plant life.

All in all, we’d recommend you avoid this kind of lighting, as there are so many better options available.

LED Lighting

LED aquarium lighting is becoming a fast favorite among fish keepers. It’s ridiculously energy efficient and it’s unlikely you’ll ever need to change the bulbs in your lifetime.

If you buy a new aquarium, LED lighting will often come as standard, but it’s unlikely that whatever comes with your setup will be suitable for a planted tank.

You can get full-spectrum varieties of aquarium LED lights, which is what you’ll need to keep your plants healthy.

While this type is more expensive to buy than most, you’ll offset the cost long-term as it’s so much cheaper to run.

Note: You can click here to see our top recommendations for the best LED aquarium lighting for plants.

Full Spectrum Bulbs

These bulbs – also known as daylight bulbs – are specifically designed to be as close to sunlight as possible.

Not only does the light they emit make your fish and plants look as close to their natural colors as possible, it’s also perfect for a freshwater planted aquarium.

Actinic Bulbs

These bulbs give off a blue light in the precise wavelength and color temperature ideal for marine plants, particularly in very deep tanks.

They’re perfect for live coral and plants in saltwater tanks, but since you’re here looking for advice about freshwater aquariums, this type of bulb probably isn’t for you.

While they’re excellent at what they do, they’re very expensive and would be overkill for your average freshwater tank.

Metal Halide Lighting

These are often seen as the best of the best when it comes to aquarium lights, but it really depends on your setup.

Metal halide lamps are high intensity bulbs that produce more lumens per watt than any other type of aquarium lighting. This basically means they’re perfect for penetrating right to the bottom of very large, deep tanks.

If you have a smaller tank, however, they produce far too much heat.

Unless you happen to have an extremely big aquarium, save your money and go for a less powerful full spectrum variety.

Lighting Duration

A good rule of thumb is to keep your aquarium lights on for between 8 and 10 hours per day. If you keep your tank lit for less than 4 hours a day, the plants won’t even register it, and they’ll eventually die off.

Although not necessary, some people like to provide a “midday burst” for their plants, where they switch on some additional lamps or bulbs for 2 or so hours in the middle of their lighting period. This mimics the sun when it’s at its highest point in the middle of the day.

There are folks who will tell you to switch your lighting on for 5 hours, off for 2 hours and then on for another 5 hours, before switching off for a full 12 hours and starting the cycle again. This is supposed to limit algae growth, but evidence to support this is anecdotal at best. In fact, some people suggest it doesn’t help reduce algae growth and is worse for the plants in your tank, so it’s best to avoid this technique and go for a straight 8 to 10 hours.

If you’re setting up a brand new aquarium and you’re having trouble with algae growth, it can help to keep the lights on for just 6 hours per day. You only need to do this for a couple of weeks until things settle down, then you can go back up to 8 to 10 hours.

There’s no ideal time to have your lights go on and off, so choose a time that fits in with your schedule and waking hours. Although aquarium lighting is essential for plant growth, and helps the inhabitants of your tank get in a day-night rhythm, it’s also for your viewing pleasure so you can see what’s going on in the tank.

The best way to control when your lighting switches on and off is using a timer. They’re cheap and simple to use, and this means you don’t have to worry about forgetting to switch them yourself, you will have automated aquarium lighting. It’s also handy if you might not be home at the right times.

Plant Light Requirements

When you’re deciding how much light your plants need, you’re going to need to figure out your watts per gallon, as well as how much the plants you want to keep actually need.

This is how your watts per gallon translate in real terms,

  • 0 to 1 watts per gallon: Very low
  • 1 to 2 watts per gallon: Low to medium
  • 2 to 3 watts per gallon: Medium to medium high
  • 3 to 4 watts per gallon: Medium high to high
  • Over 4 watts per gallon: High to very high

The likelihood is you’ll need a minimum of 1 to 2 watts per gallon if you’re trying to grow plants that only need low light. But, if your plants need a higher intensity, then you’re likely to need a higher amount of watts per gallon.

This is where you need to deduce how much light your chosen plants need. If you’ve been told about their requirements by the person who sold them to you, great, skip ahead. But, if you’re in the dark about it, so to speak, read on.

You can usually tell how much light a plant requires by looking at the color of its leaves.

Those with dark green leave need less, as the color shows you the plant contains plenty of chlorophyll, and thus photosynthesizes more efficiently. Light green (or red) plants contain less chlorophyll, so they need more light as they’re not as efficient at photosynthesizing.

If you’re planting a variety of species with different requirements, you can use this knowledge when planning the layout. Those plants that need less light should be shaded by those that need more.


Now you’ve read this article, you should know all the basics you need to know about planted aquarium lighting for a freshwater tank.

Of course, there are many other aspects of growing and maintaining aquarium plants besides lighting, so you might have some more studying to do.

If you want to do some major aquascaping sometime in the future, you may need to up your lighting game. But, if you just want to keep a small variety of hardy aquatic plants to spruce up your tank a bit, this will provide plenty of information.

Happy fish keeping!

B Hamilton

Hey there! I'm Brian, a lifelong enthusiast and fish keeper with a wealth of knowledge and experience on freshwater aquariums, that I love to share on this site. If you have any questions or need any help, please do ask in the comments section below, I'd love to hear from you and will help where I can.

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