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#### comatoast

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I currently use a Coralife PC fixture for my tank- 65 watts, 6700K. I will soon be getting a Catalina PC which houses two bulbs, so here's my question- there are PC bulbs available (as near as I can tell) from 5000K to 10000K, and several levels of "K" in between. Does anyone know the optimum "K" or if there is an optimum combination of "K" (say one bulb 5000, one 10000), or is it hit-and-miss? (I'm almost afraid Hoppy will respond to this and confuse me further with his immense knowledge on the subject, but I'm willing to risk trying to figure it out.) As always, thanks in advance for sharing your knowledge and expertise-

#### Hoppy

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Now, I can't resist commenting on this.:hihi:

Color temperature isn't an accurate way to judge how the light from a bulb will look. A fluorescent bulb can't have a real "color temperature", so about all you can say is that the higher the color temperature, the more blue-white the light will be. But, that isn't even accurate. The GE9325K bulb gives a lavender tinted light, not a blue tinted light. So, if you take the color temperature and multiply it by pi, divide by the depth of the tank, and square the result, you will have learned to use your calculator better.:hihi:

#### kali

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lol you guys are funny .!!! this's thread just made my days

#### comatoast

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Too funny, Hoppy. At the risk of provoking further incomprehensibility (is that even a word?), I'm not all that concerned about the color of the light, except to the extent that it affects plant growth. Are there facts unknown to me about the "K" rating such as, "10000K is great for stem plants but lousy for mosses and anubias" or "6700K looks great, but promotes algae growth" or, "5000K made my entire school of neons go blind." Any and all factoids are welcome (even if my eyes cross while reading them).

#### UDGags

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Pretty much all light helps plants grow....it all depends on the plant which helps the most. I wouldn't be too concerned if you have some in the 5-10K range. The variance on the bulbs isn't going to make or break you.

Here is a few comments from previous posts/searches I did....

Blue Light is around 420nm in wavelength
Red light is around 700nm in wavelength

Actinics is 420nm/460nm wavelength (mainly blue)

At a 6,000K rating the ratio between red and blue light is equal. As you move higher say to 10,000K the blue light is higher.

Plants need both red and blue light. Algae tends to prefer yellow/red light so sticking to the 10K bulbs is a common since it limits the red light spectrum. If you used just Actinics you would mainly get blue light and not a lot of red light.Chlorophyll absorbs at the two wavelengths, red (620–750 nm) and blue (450–495 nm). Green (495–570 nm) is not absorbed (for the most part) and is reflected (hence why MOST plants are green).
The blue spectrum is fairly close to the green spectrum so you have a chance of loss unless you're on the low end....plus some actinic are rated at 420nm (Violet) so you are below blue there.

As for Algae I found this interesing tidbit on Google.

"Red wavelengths are absorbed in the first few metres of water. Blue wavelengths are more readily absorbed if the water contains average or abundant amounts of organic material. Thus, green wavelengths are often the most common light in deep water.

Chlorophylls absorb red and blue wavelengths much more strongly than they absorb green wavelengths, which is why chlorophyll-bearing plants appear green. The carotenoids and phycobiliproteins, on the other hand, strongly absorb green wavelengths. Algae with large amounts of carotenoid appear yellow to brown, those with large amounts of phycocyanin appear blue, and those with large amounts of phycoerythrin appear red.

At one time it was believed that algae with specialized green-absorbing accessory pigments outcompeted green algae in deeper water. Some green algae, however, grow as well as other algae in deep water, and the deepest attached algae include green algae. The explanation of this paradox is that the cell structure of the deepwater green algae is designed to capture virtually all light, green or otherwise. Thus, while green-absorbing pigments are advantageous in deeper waters, evolutionary changes in cell structure can evidently compensate for the absence of these pigments."

#### Gatekeeper

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To just answer your question and not give you a science lesson, a 6700K (more yellowish hint) or 10000K (more white) is usually fine. Nothing wrong with doing a combination of both lights. They last around 6 months for optimal usage and should be replaced.

#### UDGags

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hehe, that's why I started with my first sentence

#### Hoppy

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To just answer your question and not give you a science lesson, a 6700K (more yellowish hint) or 10000K (more white) is usually fine. Nothing wrong with doing a combination of both lights. They last around 6 months for optimal usage and should be replaced.
But did you take the square root of that?

Seriously, many people with lots of experience say that it is the amount of light that is by far the most important, not the color temperature. So, many people have had nice planted tanks with T12 bulbs with 4000K color temperatures, or about that. To my knowledge no one has shown us any data that demonstrates a color temperature effect on algae growth. And, if the PAR intensity is held constant, no one seems to have demonstrated that plants grow faster with any specific color temperature light. Just remember that our eyes are not very sensitive to red light, compared to our sensitivity to blue light. So, low color temperature bulbs may look dimmer than the PAR intensity would say they should be.

#### paulrw

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Hoppy, you are the only other person I've heard say"a fluorescent can't have a true color temp" i read this a couple of years ago it said if i remember it correctly, a fluorescent light can't have a true kelvin rating due to the un-interrupted color spectrum and a light source is judge on the kelvin scale were its spectrum is broken? so a kelvin rating on fluorescents is just a guess or a suggestion from the manufacturer .

#### Hoppy

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Hoppy, you are the only other person I've heard say"a fluorescent can't have a true color temp" i read this a couple of years ago it said if i remember it correctly, a fluorescent light can't have a true kelvin rating due to the un-interrupted color spectrum and a light source is judge on the kelvin scale were its spectrum is broken? so a kelvin rating on fluorescents is just a guess or a suggestion from the manufacturer .
It is hard to understand how a bulb could have a 9325K color temperature, instead of 9000K or 9500K, but the 9325 just looks like it should be a better temperature - at least that's how it looks to me. Similarly, the 8800K color temperature bulbs - why not just 9000K? As best I know, neither of those two actually look like you would expect a bulb that is 9-10,000K to look. Then I wonder, based on what people have reported, if all 10,000K bulbs are even close to being the same in light color. I would think it is all advertising hype to help sell bulbs, except that anyone can see the big difference between 6400K and 5500K spiral compact fluorescent bulbs.

If aquarium sizes were specified like this, a 100 gallon tank might actually be 70 to 130 gallons. And, a 10 gallon might be 7 gallons. And, please don't tell me that they do vary that much.

#### lauraleellbp

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I read way back when that kelvin ratings are a more subjective and therefore not regulated measurement.

#### Robert H

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And the GE "plant bulb" is under 5000k. "Full spectrum" is usually around 6500k, while "super daylights" can be anything from 7000 to 10,000 K. I'm with Hoppy, it never makes sense. Plants don't seem to care what the kelvin is. They will grow the same no matter what the color of the bulb is. I've grown plants under cool whites. T5s "freshwater" have a "pink" bulb, LEDs don't seem to have an assortment of kelvins.

#### comatoast

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My sincere thanks to all for enlightening me on this issue. Just so I've got this straight now, the consensus is that even though these ratings (Kelvin) are advertised as a somehow significant specification for PC bulbs, they're inexact and essentially unrelated to plant growth, so I should just use whichever bulb(s) I like the look of, yes?

#### paulrw

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I'd stick in the 5000k to 10,000 and avoid actinics but i think plants don't really care people say certain colors promote algae i.e.actinics and low kelvin rating bulbs but i don't believe theres hard evidence to prove this i run a 10,000k bulb and an 18,000k "pink" bulb on one tank and the growth is great and no algae problems

#### wkndracer

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6700 and 10K in combination is what I settled on to good result with T5HO.
The colors viewing the tank is good for my taste and the plants do well.

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