Pond turnover may occur at any time of the year. However, one of the most common times is in late summer, when the water is very warm and oxygen demand is already high. Turnover is often caused by a sudden cooling of weather or a cold rain that cools the water close to the surface. The cool water on top is more dense than the warmer water below, causing the pond to turn over. Turnover often releases anoxic water from the bottom, which causes a lack of oxygen throughout the pond. This can result in a large die-off of fish. Often, by the time it is discovered, oxygen levels have already returned to normal.