In the past it has been recommended to keep temperatures between 76-78F, but actual average reef temperatures range above 80F to 86F even. There is a fine line in that 84-86F will push coral growth and boost metabolism within the entire tank as a whole, but any mismanagement as far equipment failure (heater, chiller, etc) could cause bleaching epidemics. As MT79 said, 80-82F is ideal and gives room for error in case anything goes awry.