pH of 7.8 may be an issue. Remember pH is a logrithmic scale so being a "little off" can be serious stress to your fish and coral. Slowly try and raise your pH into range, and by slowly no more than +0.1 a day. If you wanna know "how to" raise your pH read Randy Holmes-Farley's thread on marine chemistry.
Also, 10 ppm Nitrate is getting kind of dangerous as well. Do a water change and see if you can bring the Nitrate closer to zero. I've read that clams like the Nitrates, but only in very small quantities. Even with a 0.0 ppm {zero} reading on Nitrates there will still be enough in there for your clams. Again small water changes over time. Don't rush to get the nitrates to zero.
Be patient, take your time, and listen to mbbuna's advice. That should help things a bit. Best of luck, keep us updated of your progress.
PS What kind of lights are you running on your tank? And do you have any sort of deep sand bed refugium set-up or other filtration method?