Types of Aquarium Thermometers

Aquarium thermometers are a small but essential item for your aquarium. Although the tank may look fine, changing or unsuitable water temperature can have a negative effect on both fish and plants. Having a good thermometer to track your water's temperature at various times and days will help you identify and correct problems due to temperature changes.

There are three basic types of aquarium thermometers: stick on the outside, float/standing in the water, and digital meters. Each basic type has several subtypes with their own pros and cons. Various thermometers are better suited to different styles and sizes of aquariums. Regardless of the type you choose, we recommend using a thermometer for every aquarium. Record temperatures regularly to ensure the aquarium water temperature is stable and in the optimal range for your fish.

Stick-On (LCD) Thermometers

Stick on thermometers are the most widely used thermometer in freshwater aquariums as they are often provided as part of the new aquarium package. They are usually referred to as LCD (Liquid Crystal Display) thermometers and sometimes called "digital" thermometers, which is not fully accurate.

Although they display the temperature with digits, it is not an electronic digital thermometer. When looking for thermometers, particularly online, make sure that you aren't sold a common LCD stick-on thermometer in the place of a more expensive digital thermometer.

LCD thermometers are accurate enough for general use, versatile, easy to use and inexpensive. Many have dual Fahrenheit and Celsius scales, while some have a single scale in larger numbers. The range also varies from model to model, which isn't a big issue for general use, as they will have a wide enough range to include the normal low and high temperatures acceptable for the fish in an aquarium. Most are designed with a vertical scale, but a few horizontal ones are available.

There is some debate regarding the effect of the room air temperature on these types of thermometers, as they are located on the outside of the tank and can be impacted by the air temperature and thickness of the glass between the thermometer and aquarium water. This is generally not a significant issue, with the exception of cases in which the ambient room temperature is much lower than the aquarium water temperature. In these cases, the LCD thermometer may read several degrees lower than the actual temperature of the water.

Usage Tips

When using LCD thermometers, do not place them in direct sunlight or near a heating or cooling vent; aquariums should not be placed in such locations anyway. Also, do not place the thermometer below the gravel level, as the reading will not be as accurate since there is minimal water flow through the gravel. Lastly, the thermometer reads best when viewed straight on, so place it accordingly. If you have to contort your body just to see the thermometer, it's not in an ideal location.

Floating or Standing Thermometers

Floating or standing thermometers are placed inside the aquarium, giving a more direct measurement of the water temperature. Available styles include thermometers that stick to the inside of the tank with a suction cup, those that hang on the inside with a clip, others that simply float around in the water, and models that are weighted so they sink and stand on the bottom of the tank.

Possibly the biggest downside of these type is the fact that most are made of glass, and thus are more fragile than other types. If your tank has large boisterous fish, this is not a wise choice for a thermometer. Another negative is that the scale is often in smaller print than other styles of thermometers, so is hard to read accurately. This is offset by the fact that many have a colored "safe" zone imprinted on the thermometer that allows for a quick visual temperature check.

Usage Tips

Like LCDs, these can be placed wherever needed, but unlike stick-on thermometers these can be moved to take readings in different areas of the aquarium. In large aquariums, multiple thermometers may be used at either end of the tank. The standing models sink, which allows for a temperature reading near the bottom of the tank.

Digital Thermometers

Digital thermometers consist of a probe that is placed in the aquarium water, either permanently or temporarily, which connects to a digital display outside the tank. Some connect wirelessly, while others have a cord that physically attaches the probe to the display.

Generally, these models require batteries to operate, although a few high-end models plug into the wall outlet. The sensor probe is usually attached to the display, which can be unsightly. A few models place both the sensor and display inside the tank. With these, there is a risk of battery leakage in the tank.

Some models provide an audible alert feature that will sound an alarm when the temperature falls below or above the desired range. Most lower priced models do not provide an alert, nor do they display in more than one scale.

Usage Tips

When choosing a style, check the probe cord length, as it varies from model to model. As with other types of thermometers, the temperature ranges that can be read also varies. Keep in mind that although these models are usually fixed in one spot, you can easily move them to take a reading in a problem area of the tank.