Up to 10,000k the bulbs get bluer which looks dimmer to us but functions fine for freshwater plants. Above that they nearly always are labelled actinic or blue which is good for marine tanks or some for night time bulbs. When you get a number value over the 10,000k spectrum you get into the realm of specialty bulbs and they may not fit a predetermined color range chart. Hagen's glo bulbs are a good example. 2 (used to be all 3) are 18,000k but aqua-glo is a reddish bulb that I use for sunrise/sunset effects because it looks dimmer to humans due to the lack of the yellow-green spectrum, narrower spike in the blue spectrum, and broad red-orange spike. While, power-glo is a very visible light with some blue to it because it has a big yellow spike, small green spike, and wide blue spectrum spike. Another is coralife's 20,000k bulb which is designed to be a very high intensity white with a very high blue spike but also plenty of yellow and green so it does not dim to human eyes. It specifically says it can be used for freshwater aquariums. 10,000k+ bulbs are not as simple as lower spectrum daylight bulbs.
Will 18,000k work as well as 6700k?
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smitty
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IME, the 6700K will work better for plants than the 18,000K. Smitty's post actually explains a lot about the different wavelengths used by plants and marine. I have found it difficult to find fluorescent lamps which give the different wavelength peaks in nm. Most give the color temp. I don't buy my fluorescent lamps at the lfs...too expensive. I buy the ones I need in the 6500K to 6700K range at an electrical supply house. They may have to order them, but they always get me the ones I want and my plants do great.
Mark
Mark
To boil all this down. An 18,000K lamp is not a good lamp for freshwater plants. Although it is very unlikely that it actually is an 18,000K lamp.
If a lamp were to have a CCT of 18,000K it would fall off the Planckian locus curve and be just a blue light. Blue light is fine for corals but not so good for plants. For the most part, corals in aquariums come from depths from 30-100 ft. At these depths red light is almost completely attenuated and the photosynthetic zooxanthellae are adapted to using blue light almost exclusively.
Freshwater plants on the other hand may be growing emersed or submerged at depths less than 15'. Because of this, their photosynthetic cells are adapted to using a blend of red and blue light. For this reason, a lamp that has a better balance between red and blue light is better. Something closer to 5000K (natural sunlight).
Andy
If a lamp were to have a CCT of 18,000K it would fall off the Planckian locus curve and be just a blue light. Blue light is fine for corals but not so good for plants. For the most part, corals in aquariums come from depths from 30-100 ft. At these depths red light is almost completely attenuated and the photosynthetic zooxanthellae are adapted to using blue light almost exclusively.
Freshwater plants on the other hand may be growing emersed or submerged at depths less than 15'. Because of this, their photosynthetic cells are adapted to using a blend of red and blue light. For this reason, a lamp that has a better balance between red and blue light is better. Something closer to 5000K (natural sunlight).
Andy
Wow, excellent smitty! You were right, I get confused by all the different graphs of different values...& then to read we can't trust the chart on some boxes, well there goes my some red, some blue not much middle theory of bulb buying IF there is a chart; "full spectrum" or "daylight" if not.
