XM 20K, do these get whiter over time?

[Myka]

All bulbs fade towards the red spectrum, which in the case of blue bulbs means fading whiter. That XM 20K won't ever match or come anywhere near the color of the 14K though. Also be aware that the PAR coming from the 20K will be A LOT less than the 14K, so some of your corals may not be happy with the light anymore. When you eventually add a 14K back on there you will have to acclimate the corals to the increased intensity.