They're probably fine, what kind of cover/hood do you have?
A cover on tropical aquariums help to control evaporation, and sometimes the air between the water and cover is very humid giving the impression that it could be too hot.
I use glass tops (versa top or perfecto are two of the brand's), but the black hoods are ok also as is having a topless tank, as long as your fish aren't jumpers.
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