# 2x 15watt T8 bulbs = 30 watt total or...??



## kvntran (Feb 16, 2008)

Hi,

Yes, if you have 2x15w that means you have 30w.
The 5500k bulb is ok for planted tank, but I'm not sure about the 18k. I think you should swap out the 18k for a higher color temp.
Many people mix 6700 with 10000

I have 28w over my 10g and I like it.


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## fishsandwitch (Apr 20, 2008)

18k for a lower bulb not higher. 5k-10k is good


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## Left C (Nov 15, 2003)

Those Hagen bulbs have weird Kelvin ratings. That Aqua Glo is OK for a freshwater planted aquarium. http://www.hagen.com/usa/aquatic/product.cfm?CAT=1&SUBCAT=112&PROD_ID=01015820021001

oatlord, you do have 3 wpg, but it really isn't like 3 wpg would be on a larger aquarium. Check out what Rex says about "Minimum Light Threshold": http://www.rexgrigg.com/mlt.html


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## AndrewH (Dec 24, 2007)

kvntran said:


> ... I think you should swap out the 18k for a higher color temp...


I think you might be getting the Kelvin scale and the spectrum scale mixed up?

18,000 K to 6,700 K would be lower in the Kelvin scale.

Blue to Red, would be higher on the spectrum scale.


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## Darkblade48 (Jan 4, 2008)

AndrewH said:


> Blue to Red, would be higher on the spectrum scale.


I thought red is lower in the spectrum (i.e. 2700K bulbs appear more red) while blue is higher in the spectrum (i.e. 6500K bulbs appear more white, while 10000K bulbs appear very blue).


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## kvntran (Feb 16, 2008)

AndrewH said:


> I think you might be getting the Kelvin scale and the spectrum scale mixed up?
> 
> 18,000 K to 6,700 K would be lower in the Kelvin scale.
> 
> Blue to Red, would be higher on the spectrum scale.


Yeah I got it mixed up there. At a glance 5500k looks larger than 18k, but I knew what he meant.


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## AndrewH (Dec 24, 2007)

Darkblade48 said:


> I thought red is lower in the spectrum (i.e. 2700K bulbs appear more red) while blue is higher in the spectrum (i.e. 6500K bulbs appear more white, while 10000K bulbs appear very blue).


Kelvin scale = red to blue

Spectrum scale = blue to red (ultraviolet = below violet infrared = above red).

The spectrum scale is measured in nanometers (wavelengths, not Kelvins). The visible spectrum is from about 400nm to 700nm (blue to red).


Kelvin scale:









Spectrum scale:










Both together:


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## AndrewH (Dec 24, 2007)

kvntran said:


> Yeah I got it mixed up there. At a glance 5500k looks larger than 18k, but I knew what he meant.


It's all good. I just wanted to make sure it went the right direction instead of making it worse


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## Wasserpest (Jun 12, 2003)

oatlord said:


> I have a 10 gallon tank with two separate single-bulb fluorescent fixtures with two T8, 15-watt bulbs. Does that essentially equal 30 watts of light giving me 3 watts per gallon, or does it just equal two 15 watt bulbs?


This is a very good question that I often ponder. You have to realize that (everything else being the same) one 30W bulb will give you more light than two 15W bulbs. At the same time, two bulbs will light your tank more evenly than one.

You easily understand that when looking at incandescent bulbs. One 100W bulb will light an area brighter than 4 25W bulbs. Or, to be more extreme, 100 1W Christmas tree lights.

To make this more confusing, the lower wattage versions of T5 bulbs for example have a higher efficiency. NO T5 gives you a few more lumen/watt than HO T5 (latter running hotter). So it becomes really hard to say which is "better", for example if you have to choose 2 NO (2x 28W) or 1 HO (1x54W).

So, using two 15W bulbs will not really double the light coming from a single 15W bulb. Just another shortcoming of our beloved Watt/gallon formula, plus some food for thoughts... :smile:


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## Darkblade48 (Jan 4, 2008)

Sorry to bring this slightly off topic, but I still don't get something...



AndrewH said:


> Spectrum scale = blue to red (ultraviolet = below violet infrared = above red).


Doesn't ultraviolet mean "Above" violet, whereas infrared mean "below" red (literally "infra" means "below" whereas "supra" would mean "above").

In essence, doesn't infrared light have a longer wavelength than red light, but a shorter frequency whereas ultraviolet would have a shorter wavelength than blue light, but a higher frequency as well (since we know that E = hc/wavelength and E = hf, thus frequency is inversely proportional to wavelength...)?

Edit:


AndrewH said:


> 18,000 K to 6,700 K would be lower in the Kelvin scale.
> 
> Blue to Red, would be higher on the spectrum scale.


Oh I see now, I usually look at the spectrum scale the other way around (i.e. from high to low wavelengths//low frequency to high frequency).


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## AndrewH (Dec 24, 2007)

Wasserpest said:


> This is a very good question that I often ponder. You have to realize that (everything else being the same) one 30W bulb will give you more light than two 15W bulbs. At the same time, two bulbs will light your tank more evenly than one.
> 
> You easily understand that when looking at incandescent bulbs. One 100W bulb will light an area brighter than 4 25W bulbs. Or, to be more extreme, 100 1W Christmas tree lights.
> 
> ...


Well if we take out most of the variables (2) 17W - T8 - 24" and (1) 32W - T8 - 48" are roughly the same amount of light @ 2510 lumens to 2520 lumens respectfully. Which is normally the case when you compare one source to itself (as in T8 to T8, just different lengths). (2) 14W - NO T5 - 24" and (1) 28W - NO T5 - 48" are 2372 & 2558 respectfully.

(2) 28W - NO T5 - 48" @ 2558 lumens each = 5116 and (1) 54W - T5HO - 48" = 4825 lumens.

With the numbers that close, I'd say it comes down to space... do you have room for two tubes or only one tube.

100 watt incandescent = 750 to 1500 lumens
50 watt incandescent = 475 to 510
25 watt incandescent = 160 to 210

Thing is... with one 100 watt bulb it occupies 2-3/8" (the diameter of a normal light bulb), but if you have 4 - 25 watt bulbs you essentially have 4 times the diameter thus spreading out the light slightly more.

Of course this doesn't take into consideration the light loss due to restrike and other inefficacies of 2+ lamps over one .




Darkblade48 said:


> Sorry to bring this slightly off topic, but I still don't get something...
> 
> 
> 
> ...


Ultra is Latin for below - Infra is Latin for above. If you look at the spectrum scale above, to get energy below violet you'd have to go ultraviolet and the same for energy above red - infrared. 

Violet/blue light has a low nm wavelength (around 450nm) whereas red has a higher wavelength (around 750nm).

EDIT: lol I didn't see your edit until just now.


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