I picked up a 175 XM 15K last weekend for the 20 gal. I had a 20 k XM that needed to be changed. I decided to brighten it up a bit. Very white with a blue hue to the tank. I really notice the blue when walking into the room before actually looking directly at the tank. Hopeing for some better growth rates.
The 15K is definately whiter and less blue than the 20K. Looks to be a good color though. To give ya a color indication, my personal taste would be about a 17K XM if that were available. I have a 175 14K hamilton on order. Will let ya know the color difference when it gets here.
just buy them from a reputable dealer so in case they yellow that you can send them back.
mine yellowed after about 2.5 months of use. they went from a nice white/blue to a yellow almost like a 6.5k bulb. i was using the xm 250w 10k bulb though. i was driving it using a pfo standard ballast.