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I was going to say the same..if it gets up to 82 I would worry. You might want to invest in a chiller or switch your lighting. At one point I had an MH set up for winter and T5 for the summer. Personally I think it makes more sense than a chiller because you are only running either fixture and not the fixture and chiller so it saves electric.I would say anything above 84 is getting into the danger zone (yes!) but it also depends on how quickly the temp swings. A drastic swing even within the acceptable range can kill things. I would try not to let it get past 82 so you have some room but watch for the swings.
above 84 i would call critical above 82 or so i would say is high. Some people keep their tanks that high but i'd considered too high.
Generally agreed with the above. I would do my best to ensure that the tank isn't rising above 84 F, though pretty much everything we keep in captivity can tolerate temps a few degrees higher than that for short periods of time without problems (hours to days, depending).
Exactly what your temp targets are depend, I think, a little on how tightly you can regulate temp. If the tank naturally swings a few degrees daily (which is fine and potentially quite useful IMO), as long as it is maxing out at or below 84 F (e.g., 79-80 F in the morning, 83-84 F at lights out), you should be ok. However, there could also be the potential for temps rising higher than expected if you get a heat wave while you're away for the weekend, or something of that sort. Rock solid temps that vary only slightly on a daily basis (e.g., 79-80 F) can be great, but corals and other organisms are likely to be hypersensitive to elevated temps in the event of equipment failure, and a spike in temps. For instance, corals that see a temp that varies from 79-83 F daily are going to be fine at 84 F (or even a bit higher for a short period). Corals that are kept at a constant 79-80 F may be significantly stressed with a spike to 84 F, and may well freak out and bleach or die at higher temps.
So, I think a little of both worlds can be useful: some temperature variation when you are around helps to condition the corals and other critters to cope better with an unexpected temp spike in the event of equipment failure or a heat wave, but you definitely want to minimize the chances for problems when you are away, as bad things snowball fast.
cj
Excellent advice Chris. You should write this stuff up sometime...oh wait......