There are two basic types of aquarium heater (and a sub type)

Another term for the automatic heater is a Non-Preset Heaters, while the Thermostatic heater is also referred to as a Preset Heater.

PRESET HEATERS:

The Thermostatic or Preset heater has a built in thermostat (or a remote sensor as is common in many titanium aquarium heaters). These heaters may use a digital thermostat, usually monitoring the impedance of a thermistor (a resistor used to measure temperature changes in the aquarium, relying on the change in its resistance with changing temperatures), or the potential across a thermocouple (a temperature sensor used to measure electric potential difference).

There are many models of aquarium heaters available today that come preset or with a temperature gauge on the side of the heater or on the control knob to help the aquarist set the temperature. The aquarist should allow the heater to adjust to water temperature for 30 minutes before plugging in this type of heater to allow the thermostat to adjust. Use these settings on the dial as a guide to make fine adjustments, as they should not be depended upon for total accuracy... Though some manufacturers make other claims, these gauges are at best accurate within two or three degrees Fahrenheit (one to one and a half degrees Celsius), and generally get less accurate over the life of the heater. An aquarist should always check their heater for accuracy and not assume that because it is set at 78 F, it will stay at 78 F.

A subtype of the Preset heaters is the Remote Sensor Heaters such as the Via Aqua Titanium heater. The advantage to these heaters is twofold; one is that they tend to be even more accurate by having the sensor far from the heating element, and two; the Titanium (and also Stainless Steel) heaters are much more durable and difficult to break especially with large fish such as Oscars.

AUTOMATIC OR NON-PRESET HEATERS (HOB Heaters)

The Non-Preset or Automatic heater has a very remedial thermostat; or better they have metal contacts that are tightened by turning the dial (metal contacts installed on bi-metal lead, which are brought together with an adjusting screw). When the heater is "turned up" or tightened, it turns on for longer periods of time. These heaters need adjustments between summer and winter. Examples of this heater are the Radiant by Hagen, or even the old Metaframe heaters. Sometimes these heater are all some aquarists can fit or afford on their tank (although Thermostatic heaters have come down in price a lot), but I have seen many an aquarist cook their fish with these, as subtle changes in the dial often over correct and a sudden ambient warm spell is often not corrected for.

With these heaters it is imperative that the water level is kept up to the proper level as marked on the heater or they may crack, it is also important to check these for seasonal variations (Summer/Winter), as these heater adjust poorly for these. These heaters should not be used in a room or garage with wide temperature swings as they will not generally adjust properly. As with other heaters it is a good idea to place the heater in the tank and wait for 30 minutes before plugging in (although this is less important with these automatic heaters). Adjustments should be made very gradually and it is easier to adjust these heaters if you start out at our near the desired temperature, otherwise you may be in for a rollercoaster of adjustments.

For further information including pictures about aquarium heaters, please see this article: 'Aquarium Answers- 'Aquarium Heaters, preset vs, non-preset'

About the Author

By Carl Strohmeyer
I have been in the Aquarium maintenance and research business for 28 + years now and in the aquatics hobby about 40 years. Aquatic Information and Resources
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