This has always been a discussion. Most places list 76-78. However most natural reefs are in the 78-82 range. The best i can up with are fish prefer the lower range. They will be ok at higher range but are more prone to infections. Corals prefer the higher range but will survive at lower. Articles i have read say corals actually grow faster in the low 80's like most coral in ocean. But since most people who have reef tanks also keep some fish the 78-80 is a middle ground.