Consider keeping the lid off so heat from evaporation can escape (lowered my tank 3-4 degrees), and even blowing a fan across the top of the water.
You can also freeze plastic bottles with water, and float them in the tank.
The biggest issue tends to be lack of oxygen in the water when the temps get high - consider lowering the water level of the tank and adding airstones to get as much extra agitation as possible.
Maybe they got lucky - I read one paper on the subject that said that fish have basic requirements as far as nutrition, water cleanliness, and temp. One of these things goes, and the fish may get by for a while, but if anything else happens, it's the straw that breaks the camels back so to speak. Meaning, additional stress, a rise in NitrAtes or an overfeeding or the temp swinging too much, and it could cause big problems such as one fish developing an illness... which then stresses out all the other fish as they try to fight it off.