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It depends on the bulb & ballast. But I've always read the best thing for corals is stablilty. That includes light spectrum - because the corals will have to re-adjust.
Radiums give GREAT color. I run radium, Phoenix & an Elos bulb. Honestly, I prefer the Radium 20k to the phoenix 14k. It's brighter, better color. Growth is about the same.
As far as 20k slowing growth - check out PPFD readings @ Sanjay's site. An XM 20k has terrible PPFD / PAR readings (whereas an XM 10k blows almost everything out of the water), but a phoenix or radium have really good readings for the color temp. Remember that light is only 1 factor in coral growth.
My advise is go with a quality bulb (that has decent PPFD / PAR numbers) in the color YOU like, & keep your parameters stable - and the rest will follow.
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