Your heater setting shouldn't matter. If working correctly, it should only come on when the temperature drops below what its set to.
As for the hot external temperature, yes, it can hurt/kill fish if it affects the water's temperature. The best way is to monitor it with a thermometer. If its above the safe range for the fish you keep (you didn't say what you have), you can direct a fan to blow across the top. The extra evaporation will help keep the water cooler.
Keep in mind that the warmer the water, the less its able to carry oxygen. It may also be a good idea to throw a couple of air pump driven airstones in there too, or if you have the 'waterfall' type of filter, allow the water to drop more to make more surface aggitation.
