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Originally Posted by Don E
Some people would definitely assert that the actinics are necessary- I'm a bit sceptical. They certainly do look good. People claim they reproduce the sun's light filtered through ocean water at typical reef depths. I'm inclined to think the appeal is more aesthetic than practical (and may have something to do with the fact that many reefers will pay a LOT of money for lighting equipment, and retailers know that...) I'm still reading about it before I make up my mind.
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Allow me to play Devil's Advocate somewhat, then. Eric Borneman, author of the very popular reference book "Aquarium Corals", made this comment in his forum over on reef central:
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Corals can use virtually all of the visible wavelengths of light and some of the near UV range to photosynthesize. The wavelengths that have been shown to be most effective at stimulating the photosynthetic machinery of most zooxanthellae species (i.e. shallow water tropical corals and other inverts) is white (composed of a mix of wavelengths) and blue (composed of a different and narrower range of wavelengths). Actinic light is a term for a bulb that produces blue and some near UV light wavelenghts.
Thus, actinic light, depending on how much of it there is, can be very useful and effective in stimulating coral zooxanthellae. You can grow corals well using only actinic light - if there is enough of it - same as any light source since corals can use all the wavelengths and will photoadapt to maximize use of whatever wavelengths are present.
That's lighting in a simplified, perhaps non-idealized but functional nutshell.
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Personally, I agree with Eric. I've noticed an improvement in coral appearance when it comes time to replace my actinic bulbs in the tank.