most plant bulbs will be in the range of 5500-10000, anything beyond that range and the benefit to plants decreases while the benefit to algae increases. if you're starting out, go with the 6700k. that's what is on my 29gal and the new 10gal right now and they look very good (the 10gal is actually 6400k.) 5500k and stuff below with tend to have a wash out effect on red plants, whereas if you go into the 9325k range, reds really start to pop out. i like how madasafish's tank looks and he's using a combination of 9325 and 10000k. you can experiment with those things, but for starting out the 6700 is pretty foolproof.