Fiona, by far the best solution is to keep the newts in a cool room, either a basement or a room with extra-cold air conditioning. I know one person that ran an air duct from their AC outlet to the side of their tank, keeping the tank always cooler than the room. But if your house is constantly over 80F, it's just not feasible to keep newts (except a few species that are more temp tolerant).
You can make several adjustments that will reduce the temperature a degree or two, for example, using a screen top instead of a hood, or moving the tank to floor level instead of higher up, or reducing the use of electrical appliances (lights and filters). And there are other adjustments that will lower the temp, but only temporarily. I put frozen bottles into my tanks every other evening during the hottest weather. For most of the tanks, this keeps them under 72F all night.