Temperature can be the single most important component in the life 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 systems out of whack.
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. Since house temperatures in the spring and fall vary so much, this is when we see the cases of ich or other diseases caused by a compromised immune system. When fall comes, an easy solution is to turn your heater down no more than one degree per day as temperatures cool. This is when a quality heater will really help you, since value heaters may cause the temperature to drop more than one degree per day, causing much unwanted stress.