I keep mine at 28-28.5 in the summer which is high but my halides heat up the water so I live with it. In the winter like now when it's crazy cold outside (gonna be -12C outside tonight) I let my tank get down to ~24C to save on electrical costs to try and heat the beast.
Unfortunately our corals, fish, and inverts tend to come from a variety of water temps across that whole range. Deep water corals are more in the 21-26C range while shallow water corals are typically found in the 25 to even as high as 32 for species found in tidal pools. Fish tend to be significantly more adaptable than corals OR inverts and can handle a significantly larger range provided there is enough gas exchange at high temperatures to get them Oxygen.
Where you get in trouble with reef tanks and temperature is first if the temp gets too high for your corals/inverts. Especially if you have deepwater species, they'll literally cook in high temperatures (or at least their proteins will). This happened to me over the summer when I forgot to turn on my Air Conditioner on a day when it was 35C and 90% humidity outside... Tank shot up to 32C in a flash and all my deepwater LPS suffered, I even lost many. The shallow water corals didn't look happy, but they didn't suffer, and my tridacnid clam which is often found baking dry in tidal pools was laughing at the heat
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Second trouble you can get into with temp is with fish. While they can handle a wider range of absoloute temps, they cannot handle swings in temp very well, especially downward swings. Downward temp swings tend to leave them stressed out and vulnerable to infections and ich. This is especailly true with Tangs or other similarly sensitive fish.
A word on cool water. When we lost power to my house a couple years ago for 3.5 days and I barely was able to keep water moving and oxygen in the tank with a car inverter, my tank temp dropped to around 13C/55F after those 3.5 days. Corals weren't happy but they didn't die. Fish got sick but didn't die. Dunno how much more they could have handled, but these animals are significantly more able to withstand the "cool" than the heat.
So my advice, keep it stable and don't let it get too hot. If you notice your heater stops working and the tank gets cool, don't freak, just get a new one and trust that they will recover