I regularly hit 90 for an entire summer, the swing covering 9 degrees within 24 hour period. Once it hit 91 and one of my SPS RTN'ed. Nothing else was bothered. So, 91 is too hot.
A swing from high 70s to low 80's is not something to worry about.
My current tank hits 85 or 86 daily. Normally it would sit at 80
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Brad
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