Yet the average pond will not increase in temperature by more than a few degrees, even on the hottest day. Even on cold days, water temperature changes slowly. This resistance gives aquatic environments a high degree of thermal stability, which means that fish, unlike ourselves and other warm-blooded creatures, do not have to waste valuable energy trying to maintain a constant body temperature. The atomic structure of water suggests that it should be a gas at room temperatures. This makes water a secure habitat for fish because it is not constantly evaporating or freezing solid.
Most substances get denser as they get colder.
For instance, on cooling, a gas turns into a liquid and, on further cooling, the liquid becomes a solid. Thus, substances become more dense or more solid on cooling. Water is unusual in that it reaches its maximum density at about 4 o C and below this temperature it becomes less dense. This has a very important consequence for outdoor fish ponds in winter. Because of this peculiarity, ice floats on water and, by forming an insulating layer on the surface, helps prevent water below from further cooling and thus freezing.