I find that treating my fish, under what seem to be similar circumstances, with Sera Baktopur direct helps to reduce the number of deaths. This is medication for internal and external bacteria. I follow the directions as stated. What is the condition of the fish fins? Stress is the biggest killer with Malawi's. I have had a fish go from healthy to dead within 5-6 hrs, Metriaclima callainos. Can only put it down to stress caused by the other fish. If a fish becomes stressed the others will hound it.