Well, the 8k would be more along the lines of a natural sun light. i think natural sunlight is around 6500k, being allot more of the yellows and reds. 20K like i said is great if you are looking for color. i would seriously consider a 10k or 14k.
Are your corals going to be frags?
i switch back and forth. as you may know the Avg life spawn of a MH
bulb is about 6-8 months with an avg daily usage of 8-10hours. In the Fall i buy a 10k bulb and come spring i buy a 14 or 20k bulb. i do this for 2 reasons
1) in the fall where i live its starting to get a little colder and the presents of a 10k bulb warms the room, and no i dont mean by heat
. just the color of the bulb makes you feel a little warmer. IMO
LOL this also lets my corals get a good 6-8 months of good growth.
2) in the late spring i switch over to my 14/20K bulb. this bulb gives off a bluer or cooler light. This time of year my lights are on much longer and i see alot of vibrant color coming to the tips of the coral. Since I have my lighting on alot more the life span of this bulb last me only about 4-5 months.
i forgot to mention.. when changing bulbs from 10k to 20k you can burn corals very easy... i use a series of screens i lay across my tank and remove one of them every other day and increase the lighting to 100% in steps of 5 screens.
day 1) 5 screens lighting on for 5 hours
day 3) 4 screens lighting on for 5 hours
day 5) 3 screens lighting on for 5 hours
day 7) 2 screens lighting on for 6 hours
day 9) 1 screen lighting on for 7 hours
day 11) No screens lighting on for 8 hours
day 13) No screens lighting on for 9 hours
day 15) No screens lighting on for 10 hours
if those 3 bulbs is what you have to choose from i would choose the 20K. Dont expect a good growth rate but you will see some.