here's the story. On a natrual reef it sometimes does get into the low 70's right, so why shouldn't our tanks, but maybe not fluctuating constantly like you have. i think stability is more important than temerpature, as long as it is below 83 and above 60. the reason for a higher temp. like 80-82 would be for faster metabolism and therefore growth rate, but faster metabolism requires more food, and with more food, more waste. Fastest growth isn't always the healtiest. Hot water also carries less oxygen than cold and fish with faster metabolism need more oxygen than fish with a slower metabolism.
As for salt- when there was only fish and no inverts it was better to have low salt. inside the body of a fish the fluids actually have less salt than in the water surrounding it. fish urinate salts out to keep this salinity. and if thier body fluids get too fresh they absorb water through thier skin. Therefore fish expend less energy when the salt is a bit below sea water. however invertabrates cannot control the salenity of thier body fluids in thier body, so they need the salenity of natraul sea water.
its really all about your opinions and what you want to keep.