# The Best Kelvin Rating for Plant Growth?



## Canoe2Can

No, there is no single ideal Kelvin rating. Color temp is based on the light emitted by a theoretical black radiating body heated to ___ degrees kelvin. The higher the color temp, the bluer the light; the lower the color temp, the redder the light. 

How well a particular light grows plants is based on its PUR, photosynthethically usable radiation. In the most general terms, plants like light in the blue and red parts of the spectrum. And color temperature is not a precise indicator of how much blue or red a light produces.

Anything in the 5000-10,000K range should work fine for plants. It's mostly a matter of personal preference. There are some tubes that are designed especially for maximizing PUR. Every one of these that I've ever seen was in the 8000-9300 K range.


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## Snazzy

so...for the T8 bulbs they sell at walmart and Home depot that say are for plant/aquarium rated at 2700k and 410lumens arent really that great for planted aquariums? 

is it better to get the "daylight" bulbs around 5600k then the above 2700k bulb?


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## mrbelvedere

A GE 6500K Daylight bulb available at any Wal-Mart or Home Depot will be a wonderful tube.


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## spypet

Snazzy, think of it this way;

2700k is late afternoon color light, a few hours before sundown.
6700k is Noon color light on a blue sky cloudless day at the equator.
(most aquarium plants are from equatorial regions of our planet)

so, if you were a plant submerged under a foot of water
what type of light do you think you might perfer...


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## Snazzy

ok thanks


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## RedIrocZ-28

Yah, the 6500k lights at Home Deopt cost like $6.59. I picked up 2 of them for my HC grow tank that I'll be starting up today hopefully.


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## Snazzy

well after having the "plant and aquarium" bulb they sell at HD(which give a red hue) i think is working pretty good  

because my set-up is a 6500k light in back and a 2700k light in front (the bulb above) the plants in front are growing fast and the ones in the back are growing a little bit slower and bending to be in more of the 2700k bulb 
the plant that im talking about is r. rotfilia(sp) i have like mabe 20 stems threwout my 10gal tank lol :hihi:


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## GraemeK

I'm interested to know more about this too. i recently changed my T8 tubes to the Hagen flora glo plant tubes which are rated at 2600k i believe. these have a orangier glow to them which peaks more in the blue and red spectrum i believe.

for my pico tank i have just changed the bulb to another 4000k one but also bought a 2700k one to try and see if that improves plant growth?

G


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## Canuck

As some have alluded to kelvin rating has more to due with appearance than "substance". Plants prefer specific spectrums and as someone in another thread pointed out you can calculate the kelvin rating from the spectrums but you can't know the spectrums based on the kelvin rating. People assume that since 5000k to 10000k most resemble daylight then it has the same properties, not true. Plant bulbs are a case in point, most people wouldn't find them attractive over their tanks but they are the most efficient for growing plants, imo.


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## Nate86

Hi there... this is my first post on the forum, but I have been a long time reader and have learned a lot of things about planted aquariums and how they work. I just thought I would share some of the information I have found from this site (and others) to hopefully help a few of you guys who had similar questions that I did when I first picked up the hobby.

In regards to the original question, I don't think there is a definitive Kelvin rating that is best for plant growth. The measurement itself is only meaningful to humans, as it generally indicates what kind of hue the light gives off as perceived through the human eye. Lower values are more red-ish in appearance, while higher values appear more and more blue.

That said, shorter wavelength blue light penetrates much farther into the water than the longer wavelength red. This may be why 6700k+ Kelvin bulbs seem to be better for plant growth, since the blue light is more readily available for the plant to use compared to the lower Kelvin lights.

However, plants actually use a broad range of the light spectrum for photosynthesis... not just the blues. This is known as PAR, or Photosynthetically Active Radiation. Basically, it is the output of light needed by plants for the process of photosynthesis. Photosynthesis actually uses the very much of the 650 to 670 nm range of the light spectrum which appears to humans as deep orange to red color. It also uses much of the 430 to 475 nm range of the spectrum (varying shades of blue), but not quite as much as the red range. The range of colors which human eyes are most sensitive to are the yellow/green range (550-620nm) is almost completely unused in photosynthesis, which is why most plants appear green in color (reflected light). It also explains why many warm/cool white bulbs are so terrible for growing plants--they mostly have the yellow/green tint that humans are so sensitive to but plants don't use at all.

So what should you look for in a bulb if the Kelvin rating isn't the best measurement to use for growing plants? I would recommend trying to find the spectral power distribution graphs for the light bulbs you plan to use. Most manufacturers of aquarium-specific light bulbs will put this graph right on the cardboard case of the light bulb for you to read. You want the graph to look like the letter "M"... good bulbs for growing plants will have a higher concentration of light being output in the 430-475nm range, almost no power in the 500-600nm range, and another large curve or spike around 650-675nm.

If you decide to buy a cheaper bulb from places like Home Depot or Lowes (which I don't necessarily think is a bad thing at all), you may have to dig a little deeper for your information. Try looking around the manufacturer's website. Many of them have the spectral power distribution data on their sites if you poke around enough.

Here are some of the graphs for the bulbs mentioned earlier in the thread:

GE Daylight bulb (6700K)









GE Plant & Aquarium bulb (10,000+K)









Hagen Aqua-Glo bulb (10,000K)








Note: The Hagen site incorrectly lists a different spectral power distribution graph with a higher peak in the 640-675nm range. This is the actual graph on the box, and I have one to prove it.

Hagen Flora-Glo bulb (Not sure what K temp, I think 6500K)









I hope this post helps someone who is trying to decide what light bulb to get!


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## GraemeK

Hi Nate,

The Flora Glo tubes are 2,800K as this is on the front of the packaging i have right here.

Graeme


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## gpodio

Although I mostly use 5000~8000K bulbs today, looking back, I feel some of the better (quality) growth I've had to date was under a set of Philips 3000K bulbs I mistakenly purchased with my first ahsupply kit several years back.

As others mentioned, it's the PAR/Spectrum that counts, you can have 2 same Kelvin bulbs with very different spectrums between them.

PS. I've always been a big fan of the GE AquaRays... although I've never believed them to really be rated at 9325K (way too red...) 

Giancarlo


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## fishyjoe24

mrbelvedere said:


> A GE 6500K Daylight bulb available at any Wal-Mart or Home Depot will be a wonderful tube.


how many of these GE 6500k daylight bulbs would i need on a 55 gallon tank, or a 110L(might be getting it this weekend) and are the bulbs t5,or t8's.
i've got a 260watt pc fixture, and i'm looking at 35-65 dollars for each bulb and i need 4 bulbs.

or do you think 2- 96 watt pc 6,700k bulbs would work, and just get a new ballast?

or would 10000k daylight bulbs from petsmart work. a friend is telling me that his baby tears are growing like crazy in his tank, with 10000k pc bulbs.


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## Hoppy

If you have good CO2 concentration, with good water circulation in the tank, and good water surface ripple, almost any color temperature bulb will give you good plant growth, as long as the intensity is at least in the low-medium intensity range. You will enjoy the appearance of the tank more with some bulbs, and that is a personal preference, but the plants won't grow much differently.


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## fishyjoe24

Hoppy said:


> If you have good CO2 concentration, with good water circulation in the tank, and good water surface ripple, almost any color temperature bulb will give you good plant growth, as long as the intensity is at least in the low-medium intensity range. You will enjoy the appearance of the tank more with some bulbs, and that is a personal preference, but the plants won't grow much differently.


i don't have c02 yet, my filter is a hob marineland 250 canister.
so do I need 10000k 6,700k or a mix of both 6,700k/10000k?


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## Nate86

Depends on what you're trying to grow. I have two 15-watt Aqua-Glo bulbs and two 15-watt GE Sunshine bulbs over my 55 gallon tank. I have anacharis, anubias, crypt wendtii, and banana plants in my tank and they are all growing quite well. I also have a DIY CO2 reactor as well, though.

And if you read a few of the previous posts, you'll see that the Kelvin rating is not very important when determining which bulbs to pick. It's all about the PAR spectrum of the bulb.


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## stewardwildcat

Hoppy said:


> If you have good CO2 concentration, with good water circulation in the tank, and good water surface ripple.


Doesn't surface ripple lead to CO2 loss and O2 gain? I would think you would want minimal ripple but still good circulation. Please advise.


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## hbosman

Surface ripple helps oxygenate the water and helps eliminate the surface scum that often builds up in planted aquariums. Surface ripple will contribute to co2 loss so, you don't want to go overboard. If you are using pressurized CO2, it's just a matter of turning up the CO2 a notch to compensate.


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## Hoppy

If you are using high light you need to keep the CO2 level very near the maximum the fish can live with, and they can live with more CO2 if the water has lots of oxygen dissolved in it. So, it is very desirable to have good surface rippling to maximize the amount of dissolved O2. And, with high light, you have to use pressurized CO2, so you just increase the bubble rate to compensate for the increased loss of CO2 from the water surface. CO2 is cheap.


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## stewardwildcat

Gotcha, I didn't know that you could remove the surface scum by having surface ripple. I will have to adjust my returns when I get home. I must remind myself to keep my windows cracked with CO2 being pumped into my apt.


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## FSM

stewardwildcat said:


> I must remind myself to keep my windows cracked with CO2 being pumped into my apt.


The amount of CO2 released in a day with the pressurized CO2 is basically nothing relative to the volume of air in your apartment.


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## elvisr29

FSM said:


> The amount of CO2 released in a day with the pressurized CO2 is basically nothing relative to the volume of air in your apartment.



Hahahhah


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## JeffE

This was 4 years old since last post...


- Jeff


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## gus6464

miles520 said:


> For Plant growth, maybe we can check this link ,http://www.ecosunlite.com/e_productshow/?29-King-Par-LED-Grow-light-29.html


Haha that light is basically "the sun" at <15".


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## Aqua nut

I've been growing aquatic plants for 17 years now. All 3 of my tanks have t5 dual light fixtures. 1- 10,000k in the back, 6700k in the front. Works well for me.


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## dkfishtank

Snazzy said:


> is there an ideal exact kelvin rating that you should get to get optimium growth? leaving out any opinions on what the light looks like but accual data that supports a rating? i know like 5000K to 10,000K will groe plants but is one better then another?


i have two planted tanks and my t5 fixtures for each tank have 5500K and my plant growth is sick. Remember, your substrate has to be good quality and the use of ferts should be based on the healthiness of the plants.

Bump: i have 2 planted fish tanks and I have been using a double t5 light on one and 2 dual t8 light fixtures on the other. The bulbs are rated at 5500K and have been awesome. Remember, substrate is a big issue and you something that is quality and also the use of ferts should only be used based on the healthiness of the plants.


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## essabee

The Kelvin numbers labeled on modern bulbs are merely indicative of the general colour of the light output and not its spectrum - the bulbs spectrum can be very different from that represented by Kelvin number. I realised it and tried to compensate by using several bulbs (actually 10) in my canopied large tanks to create a more solid spectrum - like that of the sun - from the spiky spectrum modern bulbs have.

With the advent of the high output LED we are moving towards a large number of light producing units even over small tanks. Although these LED have the same problem with their spectrum (limited spiky spectrum), the large numbers that we can use and a large number of varieties we can choose from - will one day ensure that we can have a reasonably solid sun-like spectrum over our tanks. I have come across posting of DIY hobbyist who are attempting to create exactly that with mixes of different wavelength LED outputs.

Once that starts happening we would be able to compare notes on Kelvin rating instead of going high brow over stimulated 'K' labels of modern light bulbs. Till then I pass...............

Like Hoppy said above +1


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## jeffkrol

essabee said:


> The Kelvin numbers labeled on modern bulbs are merely indicative of the general colour of the light output and not its spectrum - the bulbs spectrum can be very different from that represented by Kelvin number. I realised it and tried to compensate by using several bulbs (actually 10) in my canopied large tanks to create a more solid spectrum - like that of the sun - from the spiky spectrum modern bulbs have.


LED's can be quite smooth (in relation to most other common lights)
Violet emitter w/ phosphors:


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## Kobey

To add something to Jeff's images this might also help visualize the difference.

Plants really prefer the blue 450nm and a bit less wavelength on the low end and the 440-460nm wave length on the high end.

They really don't need much in-between. If you look for "grow lights" you will find bulbs with only red blue combinations.

Now "we the people" are used to seeing things in daylight so we prefer the middle area of wavelengths. I've included the chlorophyll A-B chart as well as an example spectrum. 

But the plants don't need all that extra light...we want it so the tanks themselves look good to us.


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