From what I read, the average temp of natural reefs is normally around 77 degree F. People keep tanks higher because it promotes faster growth. So your 78 isn't too low at all.
I try to keep mine at 77. I've had trouble with SPS getting RTN when I've kept the tank at 80+ degrees. At 77 I don't get as much possible growth, but I haven't lost any corals since I've lowered the temp.
BTW, great tank, its been fun following the thread!