Mine was a model citizen at first but now he is a bad boy. He will go down the row and nip my xenia 5 or 6 times as he swims by. He has picked at my clams, zoos and fights with my other fish. The good news is he hasn't done any damage to anything in my tank. I have so many corals that he doesn't...
A derasa is a very hardy clam. Check calcium, alk, magnesium. When parameters are correct they will grow extemely fast. If your deresa doesn't do well then I definately would not try any other clams especially with your current lighting.
I have fed my tank on and off. I have found that acros color up and seem to do well when I don't feed anything. When I feed too much some corals start to darken and move to the brown side probably due to the water quality suffering. As far as the xenia I don't think they need to be fed. They do...
I spot feed mine twice on the weekend with mysis early in the morning when the lights are still out and the tentacles are extended. They keep a nice color and stay nice and full this way.
The tank is 6'. I actually gave all my different varieties of GSP to my son because I read it doesn't play nice with sps and I want to start to get away from the softies and get more sps.