Another reason these heaters may be avoided is the temperature is not always accurate. Do not trust the temperature adjusting dial on your thermometer. Always use a thermometer. If you are concerned about your aquarium temperature. Buy two or three cheap thermometers and take the average of all of them and adjust your heater accordingly. You will find that most heaters will be inaccurate and you will have to adjust the dial according to your thermometer. The bi-metal heater has been known for large temperature swings that are too great for most serious hobbyists.
These heaters can be made from a couple of different materials. A more popular choice being titanium. The reason for this is the titanium heaters will not shatter under extreme temperature differential. As well as being more resistant to drops.
Plastic is another choice hobbyists have chose over glass for its strong design. Plastic heaters can come in a variety of shapes to fit your unique aquarium.
Built-in Thermostat Heater
Most heaters should be replaced yearly before they fail. In a hobby where you're spending hundreds or thousands of dollars, don't risk your coral or fish to save less than $75
Glass Bi-Metal Heaters : This is the oldest design. Which you might think is bad but the lack of technology actually makes it a reliable product. These heaters are controlled using a bi-metal thermostat. A bi-metal thermostat works by using different metals that bend at different temperatures. When the metal bends a connection is made or lost that will turn the heater on and off. Hobbyists with expensive fish, corals and plants avoid this option most of the time due to that fact that the metal contacts may stick together over time. If the contacts stick together the heater cannot turn off and is likely to cause serious damage to the aquarium if not caught soon enough.
Glass Bi-Metal Heater
Built-in Electronic Thermostat Heater : This heating element is controlled by a digital thermostat either inside or outside of the heater. Hobbyists believe these heaters to be more reliable and to fail less often. Electronic thermostats enable the heater to keep the aquarium at a constant temperature, without huge temperature swings.
No Thermostat or Built-in controller: These require external controllers to operate properly and should only be used in unique situations.
Titanium Bi-Metal Heater