Just to give you a little more info. I just got back from a LFS here in Louisville. On their SPS/clam tank he used to run 10K XM's. Then he switched to 20K XM's. But today when I went in he switched back to the 10K XM's. He stated when he had the 20K's he was selling clams like crazy, but the sps were going very slow. He said he had better coloration under the 20K's, especially on the clams-but growth of all corals was much slower. He also said the SPS polyp's just weren't as extendend as they were with the 10K's. This is because the 10K XM's have about twice the PAR a 20K XM does. However I will say his lights are at least 18 to 24 inches above the water. Again, it's a trade-off, color vs. growth(extension).