The weekend promises to become very hot, in the Netherlands it might reach temperatures unseen after 1994. I try to reduce warming up of the fish tanks in my wooden fish house as follows. All tanks, even those without fish, are filled up with fresh water as much as possible. A ventilator powered by a solar panel assures air flow throughout the day. All windows are screened, so that no tank receives direct sunlight. Tanks are covered with plastic panels, such that evaporating water drips into the tanks again.