Word of advise on the tums. Unless you fast them for a few days it is best to place the tums in the filter outlet and let it dissolve. If the cray aren't hungry enough to wolf it down immediately or you don't have enough shrimp to eat it all you will wind up with a white hairy type algae / mold that takes a little cleaning up.
I'm glad the remaining one is digging the duckweed though. I may try the tums thing.