actually it all depends, many species of fish and inverts live in cooler waters, many in warmer waters, the oceans temp varies greatly from pole ot pole, and most fish are from any excetp the poles
and like i said, for me to keep mine at 78 is difficult, it is much easier for me to raise it to 80 or 81 to keep it more constant then to have it fluctuate, which is not as good, IMO