There could be a few things. One is they need cool water. Room temperature is probably ok, but it depends on the species of bivalve. That would take awhile to kill it, so that could be. The reason he probably did die though is that they're all filter feeders, and unless you were putting in baby artemia or some form of FW snow he probably starved to death, which also would take a while. It's probably a combo of the two.