When choosing the best aquarium thermostat, the main things to consider are its cost, reliability, accuracy, and whether it will be used with a single tank or multiple aquariums. Aquarium thermostats are available as individual units as well as in models that combine a heater and thermostat in the same unit. These combination models are usually the least expensive and simplest to operate, but might not be as precise or reliable as separate units.
In some cases, the aquarium thermostat and heater are encased in separate tubes so they can placed in different locations, ensuring more even water temperatures throughout the tank. Look for combination thermostat-heater units that have a pilot light, indicating at a glance that the unit is operating. Periodically confirm the thermostat and heater's correct operation by checking the thermometer to verify the water is within the range set on the aquarium thermostat.
For a number of tanks, it may be helpful to choose to buy a single, high-capacity aquarium thermostat which will be placed in the smallest tank since it is most subject to temperature fluctuations. Each tank is then equipped with a heater of an appropriate wattage for the aquarium's size and connected in series to the thermostat. Providing a heater that delivers five watts per gallon is a good rule of thumb in the average home. A 50-watt heater should be adequate for a 10-gallon (approximately 38 liters) tank.
Most aquarium experts agree that choosing the cheapest thermostat is unwise since it may fail with disastrous results for the fish. Cheap thermostats are notorious for becoming stuck in either their open or closed positions. When the thermostat remains stuck in the open position, the water temperature will usually fall, depending on room temperature. Stuck in the closed position, the thermostat will cause the heater to operate continuously, raising the water temperature to a level that is potentially lethal to the fish.
Look for thermostats that include an adjustment screw that provides control over the range of temperatures that trigger the thermostat's open and closed positions. Better quality thermostats offer finer control over the upper and lower temperature limits. It is a good practice to verify that these upper and lower limits are within the range required by the fish by checking the aquarium's thermometer.
When shopping for an aquarium thermostat, it can be useful to read online reviews by fellow enthusiasts. Reading their experiences may help pinpoint the brands and models with the best durability, accuracy, and reliability. Online research will also help you choose the thermostat that best matches your budget.