Fish and crayfish also depend on water temperature because they are ectothermic, meaning heat is obtained from their external environment. The animal's body temperature will therefore generally be the same as that of the surrounding water. Temperature therefore has a direct effect on important factors in an organism's system such as growth, oxygen demand, dietary requirements and efficiency of food processing. The optimal temperature conditions will depend on the species of organism, but if the temperature conditions are too high or too low, it could lead to the death of many species. Another factor that affects fish is pH. The pH is between 0 and 14, with a pH of 7 being neutral. A pH below 7 is acidic and a pH above 7 is basic. An optimal pH range is between 6.5 and 9, but it will vary slightly depending on the species living in the area. The pH varies depending on a number of factors such as the number of aquatic life in the pond, the carbon dioxide they produce will have an acidic reaction in the water. The pH of ponds will therefore become more basic over the course of the day, as phytoplankton and other aquatic plants remove carbon dioxide from the water through