I believe 10K's are too yellow for most people, so I would hate to see what 6.5K's are! I personally have tried 20K's and 14K's. I must say I am not that impressed with the 14K's (even though they are Phoenix, so they are more blue). I think I saw about the same growth under both (granted I keep zoas and LPS... no SPS), but my LPS and anemones seemed happier under the 20K's.