[DrHank]

Actinics produce light in the blue spectrum. They used to be an absolute necessity for good coral growth. With the advances in modern lighting (10K, 14K, 20K) the depth of penetration has greatly improved. The K or degrees Kelvin refers to the color temperature or spectrum of the bulb. The higher the K, the deeper the light penetrates and the bluer it appears. In a good bulb, all will be equally bright but will penetrate to different depths.

Many people still use seperate actinic lighting because it increases phosphorescence and makes the corals look better. I personally like a MH in the 14K spectrum (I like the shimmer) and supplementation with T5 or PC actinics. I'm sure that many others will have their own (and different) opinions. I hope this helps.