Floating or standing thermometers are placed inside the aquarium, giving a more direct measurement of the water temperature.
These have a coloured "safe" zone imprinted on the thermometer that allows for a quick visual temperature check. In large aquariums, multiple thermometers may be used at either end of the tank.
Pros
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Inexpensive
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Can be placed in multiple locations
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Not impacted by ambient room temperature
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Many have easily viewable "safe" zone