Many species of fish can only live in a narrow temperature range. Aquarium heaters increase water temperature to a healthy level for the fish in your tank. The proper temperature for an aquarium heater will vary, depending on the type of fish. Tropical fish need warmer water. Temperate fish, on the other hand, don't require temperatures that high.
Water Too Cold
The water in an aquarium could be too cold if the bimetallic thermostat has failed with the switch in the open position. For the heater to heat, it must have electric current. The bimetallic strip closes two contacts when the temperature reaches the bottom of a preset range. It opens as the water temperature increases, regulating the temperature. A thermostat that fails with the contacts open prevents current from reaching the heating element, allowing the water to cool down too much.
Water Too Hot
In some cases, a bent or broken bimetallic thermostat may keep the contacts closed, causing the thermostat to continue to heat the water in the aquarium past the intended maximum temperature. You may be able to recalibrate a thermostat that still works in such situations by measuring the desired temperature and resetting the physical temperature knob. However, if the metal breaks with the thermostat closed, it can eventually cause the aquarium water to boil and can not be fixed.
Electronically controlled aquarium heaters are more expensive and more reliable than bimetallic thermostatically controlled heaters. However, electronically controlled aquarium heaters can still fail. When they do fail, it's usually a complete failure of the electronic control circuitry, meaning they will stop working entirely. This results in a drop in water temperature. Electronic heaters typically never fail in a way that causes the water temperature to rise.
Some electronically controlled aquarium heaters use probes with metal tips to sense the temperature. Although freshwater aquariums have less of a problem with the corrosion of these tips, saltwater aquariums can corrode certain alloys. Look for a aquarium heaters with titanium probes or some other metal that won't react with the saltwater in your aquarium.