What fish do you have in there? A lot of tropical fish can withstand higher temps then what is "recommended", such as bettas- they can live comfortably and just fine in the lower-mid 90s. 84*F is pretty common, especially for breeders, for a lot of tropical species.
As for cooling it down.. for tanks that size I really have no idea at this point- fans just push air around, but doesn't actually cool it.. floating bags of ice will cause the immediate surrounding water to get really cold way too fast, but does nothing for the rest of the water. I suppose for now all you can do is watch.. and if you have species that can not live in that temperature (such as goldfish) you may look into placing them into smaller tanks for the time being, as usually they can be a little bit cooler..