um, I think he was referring to the total watts is less at 20k as the light isn't as intense and doesn't heat up the water or house as much therefore not requiring as much wattage to cool it.
i'd use 10k if was xm or hamilton with actinics, or just use 150w iwasaki aqua2 20k. jUst put some in yesterday. Much brighter then the 14k that was in it. Looks the same in color as my t5 that is a single 10k and a single 03 actinic. Granted MUCH brighter and has the shimmer of halide.
the t5 is a normal, not the ho, so not really fair on the brightness scale.
Most would want a darker shade of blue, but if you've seen xm 10k and hamilton 14k, the aqua 2 i'd say is close to the light blue you see on the 2 sided aquarium backgrounds where the light blue is solid, at least to my cone challenged eyes..