Originally Posted by mr_X
I have no doubt that LEDs can grow corals. I use LEDs over my tanks.
What I am doubting is that Those 10 and 20 watt floods can sustain corals.
I can say from first hand experience that they can. I setup a 10w royal blue and a 10w 6000K daylight in my brother's 30 gallon refugium (my old reef tank is now his sump).
Initially they were just for growing cheato and that's still the primary reason he has these lights over his sump. After fragging some duncans and green toadstool leathers he decided to try to grow a few of them in the sump. He found the frags grew nearly as well under the 10w lights as in his 75 display tank that has 2xT5s a 4 foot LED
light with 3w LEDs (I forget the total wattage but I think it's 220w). He's using the sump partially as a frag grow-out tank and it's working out okay, although the corals colors are NOT as nice with the floods. No surprise there.
I probably wouldn't use these floodlights as the primary lights on anything but a small display or frag tank, but for a sump I can't find a real fault with them. They grow plenty of cheato as well as coralline algae and the corals are growing shockingly fine though not vibrant. I can't say (and I honestly doubt) that they would be all that useful for SPSs but that's just conjecture. He was actually only using the royal blue flood for around a month without much difference (good Duncan, toadstool and zoa growth).
I think most moderate to strong lights can grow soft and LPS
corals but the floodlights don't seem to make corals look their best. I'm sure if enough effort was put into trying out all of the different emitters a good combination could be found. I have 6000k, 10000k, 20000k, royal blue and power blue 10 watt chips but haven't used them all over the sump for any real amount of time. The main purpose of the daylight unit is cheato and the 6000k light seems like the best option. The blue is being used over base rock to spur on coralline growth.
Putting frags in there is just an experiment. I know this isn't much to go on to address the question of whether or not 10w floodlights can grow corals. Seems they can grow some well enough but for a real mixed reef I'd still go with a nice dimmable fixture made for the task.