Fish Tank Lighting (What is PAR?)

by Kevin

Nano aquarium light for planted tank

When aquarists get serious about growing plants, I mean really serious, like reefer serious, they will inevitably get into discussions about PAR.

What is PAR?

PAR stands for Photosynthetically Active Radiation (eyes glaze over ).

Radiation you say?! Not that kind of radiation.

Photosynthetically Active Radiation is simply the region of the electromagnetic spectrum utilized by plants to photosynthesize (400-700 nanometers).

For reference, light visible to the human eye is about 380 to 780 nanometers, so plants use most of the visible light spectrum in photosynthesis.

How is PAR Measured?

With PFF of course! ( eyes begin rolling back ).

PFF stands for Photosynthetic Photon Flux, which is a metric that tells us how much PAR a light emits.

One more thing, PPFD, Photosynthetic Photon Flex Density, (audible groans) measures how much light is hitting a particular location in the PAR region, like the bottom of your aquarium.

If you have a PAR meter, it measures PPFD.

What About Lux and Lumens?

Lux and lumens describe the brightness of a white light. Generally, these metrics matter when you want to buy a bright light for your driveway, like a flood light.

Lux is a measure of how much light falls on a particular surface (ex. How much light reaches the end of your driveway).

Lumens are how much light is emitted from the light source (ex. how much light is emitting from the flood light).

Lux and lumens are useful for describing how bright a light source is, but they do not tell you if your aquarium plants are getting adequate light for photosynthesis.

We humans tend to see green and yellow lights are brighter than red and blue lights.

But for plants, green lights do not seem bright because that light is being reflected away.

To accurately gauge whether our plants are getting adequate photosynthetically useful light, PFF and PPFD metrics should be used to measure PAR.

In contrast to lux and lumens, PFF and PPFD measure “how many photons (light) are available to the plant for photosynthesis?”

Why Do Plants Need Light?

Plants need light to photosynthesize, which is how plants eat and grow.

Chlorophyll is a pigment used by plants in photosynthesis, and is the most plentiful plant pigment.

Chlorophyll absorbs light particles (photons), helping the plant convert carbon dioxide and water into sugar and oxygen, fuel the plant’s growth.

Chlorophyll is most efficient at capturing and processing red and blue light.

Other pigments in the plant permit collection of some green light, but most green light is reflected away from the plant, which is why plants look green to the human eye!

If you are growing red or brownish plants in your aquarium, green light will be more effective and red will be reflected away.

So, for most planted aquariums, blue light (400-500 nanometers) is generally the most important, followed by red light (600-700 nanometers), and finally green light (500-600 nanometers) if you plan to keep red plants.

Why PAR Matters to Aquarium Plants

As mentioned before, PAR matters to plants because it tells us how much light is available for plants to use in photosynthesis.

But, not all lights with a high PAR value will provide adequate light for your plants.


Because PAR measurements treat red, blue, and green light as equals.

This means that two different lights can have the same PAR value but have wildly different effects on your plants.

For example, a purely green light could have the same PAR measurements as a purely blue light, but your plants would see little benefit from the green light.

Aquarium Lights Do’s and Don’ts

1) Don’t compare aquarium plant lights using the metric electrical watts.

Why? Because electrical watts measures how much electricity a light uses, and that metric is useless for determining if that particular light will grow plants.

A high wattage light may consume a lot of electricity but provide weak lighting conditions for aquarium plants.

Looking at total watts, or watts per dollar, may help you save on your electricity bill, but those metrics will not tell you if the light is suitable for growing plants.

The same is true for LED aquarium lights.

Whether you have a 3 watt or 10 watt LED, all you know is the input of electricity, but what matters for plant growth is the output of a light.

What you want to look for how much light is delivered to the area in which your plants are growing (PPFD).

2) Don’t use lumens to pick out an aquarium light.

As mentioned before, lumens are a measurement of how bright a light appears to the human eye.

While a particular light may look white in color and intensely bright, these lights do not necessarily provide adequate light for plant photosynthesis.

The mechanism for how we humans perceive light and how plants utilize light in photosynthesis is different.

A seemingly bright white light may actually be deficient in red and blue light because it is cost effective for manufactures to create lights using the fewest colors possible.

What humans perceive as bright light does not correlate with how plants perceive and process light.

So, a seemingly full spectrum LED aquarium light (mimicking all the colors of sunlight) may actually be a combination of blue and yellow light or a mix of blue, green, and red lights.

3) Don’t be fooled by “special” grow lights

Determining whether a light promotes healthy plant growth is both complex and confusing.

Sadly, many retailers and manufactures do not provide much help and sometimes market “grow lights” that are suboptimal for a beginning aquarist.

Online advice about LED aquarium lights also varies but if you do your research you should find a light that will work for your particular setup.

Keep in mind that many people are anti green light because the conventional wisdom is that plants reflect away all green light.

And while many plants only utilize minimal amounts of green light, red plants likely utilize a good deal of green light.

So, be wary of lights that claim to be the ideal grow light because they do not contain green wavelengths.

What you want to look for is a combination of blue, red, and green, with an output adequate enough to grow your aquarium plants.

If you don’t want to run your own tests on lights, look at what other aquarists are using on their tanks.

PAR Takeaway

If you want to know, at a deep level, if your plants are getting optimal light for photosynthesis, you will need to measure the PAR of your light fixture in the area where your plants are growing.

But, PAR meters are expensive, with good ones costing many hundreds of dollars.

If you thought your significant other was annoyed by you buying another aquarium, just wait till you break the news that you spend a couple hundred on a device that measures light for plant photosynthesis, it could get ugly!

For the Aquarists Not Backed by Investment Bankers

If you want an effective LED aquarium light, without worrying about testing PAR, I have two solutions for you:

1) Ask other aquarists for recommendations.

Your local aquarium club, local fish store, and online forums are great places to ask about what works for planted tanks.

2) Experiment.

Find a couple LED lights in your price range on Amazon and set them on your tank.

Yes, the light you buy may grow nothing but algae, but there are a few inexpensive LED lights on Amazon that easily grow low lights plants.

LED Lights vs T5 Lights

If you purchased an aquarium light in the past 5-10 years, you likely bought a T5 light.

T5 lights are a style of fluorescent bulb lights that have a diameter of 5/8 of an inch.

T5 lights use gases such as mercury, xenon, argon, krypton, and neon, in combination with tungsten electrodes to produce an efficient and long lasting light.

T5 lights are great at growing aquarium plants and multiple bulbs are often used to achieve an optimal light spectrum.

But, LED light technology has made significant advancements in the last few years and provide a number of advantages over T5 lights:

LED Advantages

• LED lights direct light downward (scattering less light) and use less watts of power, making them more efficient and cost effective than T5 lights.

• LED lights generally have a longer lifespan than T5 lights. T5 lights are estimated to last about 20,000 hours, while LED light range between 30,000 and 100,000 hours.

• LED lights produce no significant heat. While T5 lights produce only small amounts of heat, LED lights produce next to no heat and this can be an advantage when lights are placed close to plant leaves.

• LED lights do not use harmful gases and chemicals in their construction. LED lights are recyclable and far more environmentally friendly than T5 lights.

Two LED Aquarium Lights That Have Worked for Me

1) The Finnex StingRay LED Clip Light

I purchased the Finnex StingRay based on a recommendation I heard on the Real Fish Talk podcast (thanks Cory).

My goal was to find an affordable planted tank light that would actually grow plants.

Light Spectrum

The Finnex StingRay clip light boasts 12 white/daylight LED bulbs (7000k); 3 actinic blue LEDs; and 3 red LEDs (620nms).

This combination is great for plant growth (based on my research), but remember, light spectrum alone does not tell us how much light is actually reaching the plants.

I have the Finnex Clip Light installed on my 3 gallon MarineLand Contour 3 and my Anubias plants are thriving under this LED light.

This light works on my setup for three main reasons:

1) The Finnex StingRay clip light provides plenty of blue light, which plants utilize efficiently in photosynthesis.

2) In my 3 gallon cube tank, only about 10-11 inches of distance exist between the light source and the base of my plants (at the substrate level). As plants grow taller, this distance decreases even farther.

3) I have planted my 3 gallon tank with primarily low light tolerant plants. One of the plants— Hydrocotyle sp. Japan—actually preferd moderate to high light with CO2 injections, but observing its growth under the Finnex StingRay LED clip light shows me that the light is surprisingly effective.

There are actually other larger Finnex LED lights models and given the performance of this little light so far, I would not hesitate to try them out.

2) The Hygger LED Clip Light

I stumbled upon this light when searching Amazon for affordable LED aquarium lights.

Many non-name brand aquarium lights on the market suffer from quality problems, but the Hygger LED had great reviews so I decided to test it out.

I noticed immediately that the Hygger LED light provides a lot of light.

With a total of 30 LEDS in this fixture, it easily illuminates my 7 gallon Aquatop aquarium.

Light Spectrum

The Hygger LED light offers 24 cool white LED bulbs (6000k-7000k) and 6 blue LED bulbs (465nms).

The Finnex Clip light uses 18 individual LED bulbs while the Hygger employs 30 individual LEDs. (almost double).

The Finnex Clip light has 3 dedicated red light LEDs, while the Hygger only provides daylight and blue light LEDs.

Visually, the light emitted by the Finniex Clip light is softer, with a warmer hue, and easier on your eyes, while the Hygger light is bright and has a bluish hue.

Which Light is Best:

So far I am happy with both lights.

Both are capable of growing low light tolerant plants and likely moderate light plants as well.

If your tank is deeper than 12 inches, I would suggest going with the Hygger because its 30 LEDS will provide your plants with plenty of photosynthetically useful light.

If you are interested in creating low tech nano tank aquascapes, I highly recommend both these lights.

Stay zen fish keepers.