Like I said, it depends upon what temp they were kept at under normal circumstances. My tank goes from 77-80 degrees in the winter, to 80-84 degrees in the summer, with various spikes at any given time. Being in the south, I would say the average is around 81 (maybe 82) since it stays warm for a while. If you maintain your temp at, say, 78 most of the time and suddenly it spikes to 83 or higher, you will have quick bleaching problems. Tanks maintained at more natural temps, say 82, tend to tolerate swings much better.