What's happen when the temperature becomes low? And when does it become dangerous for the inhabitants of the aquarium? Normally the first key is the time in which the temperature passes from a healthy state to a critical state; and the second key is the temperature of our inhabitants during the other days. i.e. If we have 25°C (77°F) every days, and we drop to 24°C (75°F) in a 12 hours isn't a real problem. If we have 27°C (80°F) and we drop to 24°C (75°F) in a couple of hours it could be a great problem. Nevertheless the low temperature is less critical than high temperature, because we have a great amount of dissolved oxigen. When the local temperature is low, we need to warm the aquarium. The best way is to use a heater with power equal to the volume of our tank (In US gallons the power could be roughly 4 times the tank volume). If we have 300 liters tank (80 US gallons), we need a 300 watt heater. Better if we can use 2 heaters with half of the power. In the previous example, 2 heaters of 150 watt. But if we don't keep the aquarium inside our house, or we have a low temperature at home, like below 20°C (68°F), we need to double the power. Don't worry, one heater of 300 watt for one hour is like one heater of 150 watt in two hours. We don't waste the electricity. What are your choices?