Buy your own full spectrum master liquid test kit and test yourself, the store may have got it wrong.
Hanging at the bottom and deaths are a sign of ammonia problems and/or temperature problems. How large is this tank? Was it overstocked? How long has it been running?
FYI there are no freshwater crabs available to the pet trade, the ones found are all brackish and not fully aquatic, they shouldn't be kept in community fish tanks.