I run about a dozen reefs with LEDs and import and build our own fixtures. We use a 1:1 ratio (sometimes 2:1) of 12K white and 450nm blue LEDs from 2 watts to as much as 6 watts. I am experimenting with the new 100 watts emitters, but they don't come in the higher color temperatures yet.
As Mr_X stated, the lensing will dictate the coverage area the emitter will give you. But in general, in a tank with a depth of 24", I use a rule of 2 watts per gallon LED
for soft corals, and 3-4 for everything else. LED
is about 30-40% more power efficient than equivalent MH
lamps. Deeper tanks need more of course. LEDs tend to "beam" more than other lamps do...and they emit little UV
, so some like to add a T5 actinic to cause the corals to glow more. They make UV
LEDs, but I do not recommend them as they can be dangerous to use.
Mixing a few colors in is something that you need to read up on. I don't think it is necessary, but if your building your own fixture, it may not be a bad idea. I have been growing coral under LEDs now for about three years. IMO
it is the best way to go.