Very difficult for me to answer, I'm torn between all sides of this arguement.  Sometimes I want to agree with Mark's number of 420nm.  But there are times I would lean towards the 480-490 theory.  And yet still, look how many of our corals grow in the top 6 inches of the water column.  It has certainly been proven that white light or even yellow light (like from metal halides) provides the best light.  The other side I lean towards is the theory that we are adding certain lights because we like the way the corals and tank looks, regardless of how they look in the wild (like red cardinal fish in reef tanks). 

Adam 

I agree with Jon, he said that well.  There is certainly a difference between best light for corals, and best light for human eyes.  Hmmm, which light would you rather have.  Tough question. 

Adam

ffc3, you're of course right.  But getting a concensus on what lighting is best will be near to impossible (even among the experts).  For example, if you follow the advice of Eric Borneman atinic doesn't matter at all.  It's all about the amount of light in the PAR range (photosynthesis range) and 420 nm is a very poor performer in that area.