Surprisingly this inexpensive device is often overlooked. This thermometer measures 0.75" x 4.25" and weighs a puny 0.3 ounces. Colors are used to indicate the safe temperature range for aquarium fish, but it measures far beyond this range. Three most common types of aquarium thermometer include stick-on, floating or standing, and digital. It attaches to the inside of the aquarium via suction cups. In general, aquarium thermometers are rather inexpensive pieces of equipment but help monitor one of the more critical aspects of your aquarium. Also referred to as the liquid crystal display thermometer, the stick-on variety is the most inexpensive of all three options and popular for use in freshwater aquariums. The aquarium thermometer that you choose should have a few important features, including: Thermometer Type. It’s designed to go outside the aquarium and below the waterline. You attach it horizontally by using the adhesive. The best aquarium thermometer is the one that you can rely on. The back of the thermometer senses the temperature inside the aquarium through the glass, and then it shows a digital display of the temperature. The thermometer … As mentioned previously, there are three primary types of aquarium thermometers; submersible, stick-on, and electronic/digital.