I’ve been reading through a bit of literature on corals and temperatures, and - generally speaking - 83F is considered the safe limit you don’t want to go beyond, so some people might recommend not going above 82 as a precaution.
Edit: when accounting for the slight inaccuracies of our heaters/controllers, I personally would set 82F as the limit.
That said, though, there are a number of caveats that determine what temperature is too high. To list a few:
1) the location the coral was collected from (some coral reefs have higher temperature tolerances based on their geography than others - 83F [technically like 83.6F or so] is the limit for the least heat resistant reefs, if I recall correctly; I believe somewhere around 87-89F was the limit for the most heat resistant).
2 ) temperature stability (the more stable the temperature the coral is used to, the less it’ll like changes to temperature).
3) how long the coral has to acclimate to the higher temperature (slow and steady wins the race).
4) corals can handle temperatures above their temperature limit for a limited amount of time (if the temperature is only slightly over, the corals only start showing distress/begin bleaching after a few days [four days at 1-2F over, if I recall correctly]; if the temperature is significantly above the limit, the corals will show distress/bleaching within hours).
So, as Tamberav and a few others I’ve seen here on the forums can attest, 86 may not be too hot, but - personally- if you’re going to run a tank that hot I’d take things nice and slow (likely ramping up over the course of a month or two) to get the corals there as a precaution.
