What are some good ways to keep a tank cool? A few days ago, I unplugged the heater in my aquarium because the temperature had risen to 90 degrees. My fish seems to be okay and doesn't appear stressed. I've kept the light off in fear that it'll raise the temperature. The thermometer is holding at about 84 degrees right now, but I'm worried because I don't want it over 80. The average temperature outside has been in the 80s and 90s for the past few days, and the house has been on the warm side as well, but I don't want to wake up one morning or come home from work with a floating fishstick.