The water in the pools has to be constantly filtered to keep it clean for the dolphins and the spectators, and the temperature and composition of the water has to be controlled to match the conditions dolphins experience in the wild. In the absence of a common international regulation, guidelines regarding the minimum size of the pools vary between countries. To give an indication of pool sizes, the European Association for Aquatic Mammals recommends that a pool for five dolphins should have a surface area of 275 m2（2,960 sq ft） plus an additional 75 m2（810 sq ft） for every additional animal, have a depth of 3.5 m（11 ft） for at least the minimum surface area and have a water volume of at least 1,000 m3（35,000 cu ft） with an additional 200 m3（7,100 cu ft） for every additional animal. If two of these three conditions are met and the third is not more than 10% below standard, the EAAM considers the pool size to be acceptable.