Consider This Case
Even though an aquarium heater may be set at 75°F, if a room reaches 90°F during the day, the water may also increase to 90°F. The temperature could increase even more if the aquarium includes pumps, lights, or other electrical equipment. If that night the room temperature drops into the 70s, the water in the tank could drop to 75°F. This sudden drop in temperature will compromise your fishes' immune systems making them more prone to parasitic infections such as ich. If you cannot stabilize the room temperature to prevent these large fluctuations, slowly raise the thermostat on the aquarium heater closer to that of the expected high temperature of the room or add a chiller. Whatever you do, remember that if you change the temperature of a tank, do it slowly.
Temperature can be the single most important environmental factor 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 severely affect their immune systems.
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.
Since house temperatures in the spring and fall vary so much, this is when we commonly see more cases of ich or other diseases caused by a compromised immune system. When fall comes, an easy solution is to turn your aquarium 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 unwanted stress.