got a crocea clam about 3-4in ....my tank is a 20gallon high....with 150w mh...sunpod....it's been about a month now..seems as if the colors of the clam are fading away...the clam is still attach to the rock and very responsive to light...what's the problem with the color??? not enough nutriet?? light???? the clam is about half way up in the middle of the tank...