I keep my reef tanks at about 78. That is the middle of road between 76 and 80 which is what they are supposed to be at. This is what every lfs
has told me, my reef club and everything I have read. All my coral and fish respond well to this temperature, never had an issue. My biocube used to run at about 80 due to the closed lid, I have since gone topless with it and it has dropped down to a stable 78. The 80 degrees did not affect my fish or coral in any negative way either, I just like 78, lol, I think it's reasonable easily kept.