Stable water temperature can be the single most important aspect of the overall well being of your fish. Fish, in the wild and in our aquariums, are very sensitive to changes in temperature and a sudden shift can send their immune system into shock.
A major component of temperature control is your heating unit. Choose your heater based on the size of your aquarium using 3-5 watts per gallon as a guide. Each heater can only raise the temperature of the aquarium a certain number of degrees. If the ambient temperature of the aquarium's room is colder than normal, 3-5 watts per gallon may not be sufficient to maintain the temperature your fish need.
In the summer, you may actually need to turn the heater down or off or even add a chiller. Whatever you do, remember that if you change the temperature of a tank, do it slowly. During the spring and fall, day and nighttime temperature in your home can vary dramatically, so it is not uncommon for aquarium fish to experience ich or other diseases caused by a compromised immune system. During autumn, pay particular attention to heater maintenance and operation. Monitor heater setting and actual water temperature to confirm heater performance. Water temperature readings on your aquarium thermometer should closely match your heater settings. Actual water temperature should be within ±1º of your heater setting temperature. This is when a quality heater will really help you, since value heaters may not be able to accurately maintain set temperature, and aquarium water temperature may drop more than one degree per day, causing much unwanted stress on your aquarium inhabitants. Or automate the process with an electronic temperature controller.