Then there were Two

Not so long ago we had four of these tetras, looking healthy and growing steadily. Then one vanished (I'd blame the shrimps but they've vanished too so it remains a mystery) and a week ago or so I noticed another was pretty stressed and being picked on. The communal spirit disappears when a fish is sick! Tried treating the tank but this morning that tetra was no more and that's really not good because now the male will start harassing the remaining female and she'll get stressed. It's a jungle in there!