We should to know why our fish died. There’re several reason that caused fish died: handling, environment or pathogenic organism.
Handling; Improperly on harvesting, packaging, transporting or acclimatizing could to cause fish health disrupted or even died. Usually on early time of fish stocking in new pond (e.g 1-2 weeks after stocking). As a solution, we should to paid better attention on handling or, if have been happened, we should to maintain of better water quality (e.g adjustment of proper temperature and salting).
Environment; Unfavorable pond management lead to worse water quality, e.g uncontrolled feeding without proper water exchange. The easiest way is new water flow through.
Pathogenic organism (disease); It’s important to go to laboratory for identification: fungal, parasitic, bacterial or viral. But simply, in my experience in tropical aquaculture, water warming and salting is a better way to maintain fish health state, before we get correct identification from laboratory. Just remember, correct medication depend on disease maker.