So I have a Giesmann Spectra 72" fixture, that has 3 MH slots. I put 1 radium bulb in, and 2 reef brites (all 20k). Boy oh boy, is there a real visual difference in the blue each light gives off. The Reef Brites are white to the eye, where the radiums actually give off much more visual blue. I didn't know it was that drastic, now I wish I had a few years supply of radiums...