Are you saying you're feeding your clam mysis? I hope not, because clams are filter feeders, and mysis are far too large. Also, how are you feeding your clam? In general, clams do not need to be "fed". There is generally plenty of sustenance already in the waterflow. If you do decide to feed, phytoplankton is your best bet, and it should be fed to the tank as a whole and never target fed directly to the clam. It would be the equivalent of someone trying to shove an entire bowl of oatmeal down your throat all at once.
That said, the nitrates are probably the biggest factor. Although clams do absorb nitrates from the water, they are also very sensitive to high levels just like any other invertebrate. 40ppm is way too high. I'd shoot for more like 5-10ppm (preferrably less).
Deresas are less light demanding, and often times considered "beginner" clams (if there is such a thing). I don't know if they're really any easier, though. If you don't meet the basic demands of the clam (i.e. stable water parameters, sufficient calcium/alkalinity and sufficient lighting), you're not going to have luck with any species. Before you get another one, get those nitrates under 10ppm.
7" would probably be just fine for a deresa. In general, you don't want to get clams that are either very small or very large (relative to their species). The sandbed is also perfectly acceptable. I would suggest placing a flat rock or other hard substrate under the clam as opposed to putting it directly on the sandbed. That will help discourge predation. You can also bury the rock under a layer of sand to make it look like the clam is sitting directly on the sand.
