Over the long hual I see temps of 79 to 81 to be optimal for growth of desirable species. Long term 78 and lower seems to slow growth rates. Over the 81 increases unwanted algaes. As for higher temps on corals at what temp does die off occur? It is a guess debated world wide now with coral bleaching and die offs being experienced world wide. My personal choice of 79 to 81 comes from recorded temps of reefs in the Carribean where I have snorkled and friends have recorded the temps on dives. I also feel temp fluctuations of more than 3 or 4 degrees over a day cycle are far too much for long term success. Take this as you will, but temp control of closed systems are every bit as important as many other water parameters.