. . "Too many or too large of fish added at once, overfeeding, under filtered tank, water parameter's not suited for species of fish (pH ,GH,KH,), Osmotic stress from ph adjusting powder's, ammonia level's allowed to get too high before water change,dechlorinator not removing possible ammonia from the tap ,and or the tank or decor was cleaned with detergents or other chemicals that remained in the tank" . .