We used 6500K "Day Light" Iwasaki metal halide lamps at the coral farm I managed at the late 90's with good success. Since some wavelengths (blue. green, red) are all important in promoting coral photosynthesis, it boils down to light intensity. If your PAR levels are less than, say, 500 micromole/m2/sec, you're probably in good shape in *many* cases.Dana, I don't run blues... I run ALL 10k daylight lighting... Is that going to be okay long term for the health of my corals?