Hypoxia
Water with too little dissolved oxygen for most fish, often caused by pollution or warm, still conditions.
Also: dead zone, low oxygen
Fish need oxygen dissolved in water, which holds far less than air does, and less still when warm. When farm fertiliser and sewage feed blooms of algae, the algae die and rot, and the bacteria that break them down use up the oxygen. The result is a dead zone, such as the one that forms each summer in the Gulf of Mexico. Fish die in large kills or flee. Ponds can go hypoxic before dawn in hot weather, which is why fish farmers watch oxygen closely and use aerators.