The lighting needs to be a lot stronger. The only thing I can tell you is invest in a MH light system, or rehome the clam, unfortunately. They need the strong lighting to survive. An outside light won't do anything, I don't believe its the right type of light. The Derasa is pretty hardy, but I don't think it would be able to do well under stock lighting. Sorry!! Also, I think that type of clam will outgrow your tank pretty quick. I believe Derasas get quite large, up to 20"...or maybe I have my clams mixed up.