Last summer, I ran into some concerning high temperature problems. Normally, the tank warms during the day from the MH lights, then cools overnight in a daily cycle. While I was away, no one was home to open a window at night and the house got very warm. The tank wasn't cooling down at night and each day it increased in temperature..at one point hitting over 86 degrees.
So this year, I had a heat pump installed on the house. As we have had a very hot week, it was the first test of whether this would keep the house (and therefore, the tank) under control. It works perfectly.
The tank has a daily temperature cycle of between 78 and 82 degrees. While that isn't perfect, it seems good enough.
__________________
400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank:
http://www.canreef.com/vbulletin/showthread.php?t=28436