I just bought a small piece of sun coral. I have placed it in a shady, calm area of my tank. I feed it with plankton by covering the coral with a plastic cover, so the plankton does not get blown away. During feeding the sun opens up nicely, but after feeding it would closes. And when the light is out, it would only stay open for a short time, perhaps one hour or so. Is there any problem with my sun? Does it need to be located in an area of high water flow?