if the clam is looking better in the QT then i would really be looking at the water. the poly filter i mentioned is kinda like a test kit. as it removes certain compounds from the water it will turn different colors, depending on what color it turns you can tell what is in the water and what its removing.
your PH is fine.
SG is ok, i like mine at NSW but 1.023 will work
your Alk looks fine but clams are very sensitive to Alk changes. you always want to make adjustments very slowly.
this is a water chemistry guide for clams
http://www.clamsdirect.com/forum/viewtopic.php?t=2516