85 is the danger point at which bleaching will occur, and it can happen within a few hours. I prefer to run at 79, so that I have a bit more leeway if the day is particularly hot. Now that we're getting into summer---no lids is a good thing (eggcrate can keep fish from carpet-surfing) and a fan to increase evaporation will help with temperature fluctuations. (Evaporation=cooling.) But you may have to top off more.