Aquarium thermometers are a small item, but have a big place in 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 and tracking water 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 available: stick on the outside type, float/standing in the water type, and digital meters. Each basic type has several subtypes, with their own pros and cons. Different types of thermometers are better suited to different styles and sizes of aquariums. Regardless of the type you choose, I recommend using a thermometer for every tank, and 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, 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 some may have what can be termed a digital display, the thermometer is not truly a digital one. 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 Centigrade 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. 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 temp. I've found this is 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.

When using LCD thermometers, there are a few tips to follow. First, do not place them in a location where they will receive direct sunlight, or near a heating or cooling vent. It is not recommended that an aquarium be placed in such locations anyway. Secondly, do not place the thermometer below the gravel level, as the reading will not be as accurate. Lastly, the thermometer reads best when viewed straight on, so place it accordingly. If you have to contort your body just to be able to see the thermometer, it's not in an ideal location.

Pros and Cons
- Inexpensive
- Easy to apply
- Can be placed in multiple locations
- Unbreakable
- Can be hard to read in low light
- No alert feature
- Less accurate than digitalMore »

•Floating or Standing Thermometers

Floating or standing thermometers are placed inside the aquarium, giving a more direct measurement of the water temperature. There are several styles, including ones that stick to the inside of the tank with a suction cup, ones that hang on the inside with a clip, ones that simply float around in the water, as well as models that are weighed so they sink and stand on the bottom of the tank.

Possibly the biggest negative of these types are the fact that most are made of glass, and therefore are breakable. If your tank has large boisterous fish, this is not a good style of thermometer to use. Another negative is that the scale is often in smaller print than other styles of thermometers. This is offset by the fact that many have a colored "safe" zone imprinted on the thermometer that allows for a very quick visual temperature check. Like LCDs these can be placed wherever needed, and in the case of large tanks, 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.

Pros and Cons
- Inexpensive
- Can be placed in multiple locations
- Not impacted by ambient room temperature
- Many have easily viewable "safe" zone
- Breakable
- Scale may be small and difficult to read
- No alert feature
- Less accurate than digitalMore »

•Digital Thermometers

Digital thermometers consist of a probe or probe that is placed in the tank, either permanent or temporarily, which in turn 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. The sensor probe is usually attached to the display, which can be unsightly. A few models are , with both the sensor and display being placed inside the tank. Personally I would avoid such models, due to the risk of battery leakage in the tank.

fully submersible 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. When choosing a model, check the probe cord length, as it varies from model to model. As with other styles of thermometers, the temperature range 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.

Pros and Cons
- Most expensive type
- Reading can be taken in any location with a single meter
- Not impacted by ambient room temperature
- Unbreakable
- Digital display is easy to read
- Some have alert features when temp is out of range
- Generally most accurate type
- Most require batteries to run
- Usually more expensiveMore »