Seriously---we can be serious---many, if not most, marine tanks go topless except for the light support, precisely because, especially in summer, heat is a major problem. Evaporation keeps your tank from greenhousing its way to lethal temperatures---. You'll fare best stabilizing a day and night temp of about 80 degrees, with very little variation. That's got some small wiggle room in both directions before anything would be in trouble, if you have a power-out or other problem.