Although other factors may be involved, most fish kills in the South are related to low dissolved oxygen levels.  Dissolved oxygen levels may decrease during cloudy weather (phytoplankton and other plants consume more oxygen than they produce), when vegetation dies, or when a lake "turns over".  It only takes minutes of exposure to low oxygen levels to stress or kill sport fish.  We now have tractor-powered paddlewheel and "bank washer" pump aerators available for sale or rent.  These devices—originally designed for commercial fish farms—quickly aerate and circulate large volumes of water.  If fish are seen near the surface "piping" for air, implementing emergency aeration may mean the difference between losing a few fish and major fish kill.