derasa's and squamosa's should be on the sandbed. crocea's and maxima's can be on the sandbed if it is lit brightly enough...but normally attach to rock with a byssus. The problem is, if you put them too high you will not get the best view of them, by looking down. Also, despite their need for high light, I find that they get cranky too close to the lights and hop off. I try to put them on a rock a few inches above the sand. Regardless, start them off on the sandbed until they get used to your conditions. As to how they respond when the lights go bad...decline and death. A lot of claims are made that high light organisms, like clams and SPS can't live under pc's, but I have not found it to be true. My sps grow very well...but then again, I can't seem to get LPS to do well......