The reactions to this subject can be amusing. Eric Borneman has basically staked his reputation on
actinic light being important. Dana Riddle & Miguel Olaizola wrote an article (
here) which concluded it's intensity that matters, not spectrum. When Dana tried to join a discussion in Eric's forum, Eric blasted him (her?).
I think there's plenty of evidence that actinic light works. However, the evidence that it works better than white light is weak at best (per that article).
There's an argument that blue light is more natural, based on the observation that water absorbs the longer wavelengths more quickly. However, this argument is weak for a couple of reasons. First, most of the corals we buy were collected in shallow water, where they're lit by unfiltered sunlight. Second, while it's true that blue light penetrates deepest in oceanic water, coastal water contains a lot of impurities that absorb blue. Thus, the most natual light in a deep coral reef is yellow/green (lacking both red and blue).
Part of reefkeeping mythology is that reddish light stimulates
algal blooms. I have yet to see any real evidence of that. My own experience indicates that sunlight works fine.
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