Thank you for your question. The medium used in Hagen's glass bulbed thermometers is a red dye mixed alcohol. It should not be toxic to fish. However, if the thermometer does break for any reason, it should be removed immediately from the aquarium. Although the indicator liquid is not toxic, any broken glass pieces from the thermometer can cut fish and hurt them. In addition, if the thermometer is weighted, the small balls will rust and could be a potential problem for the environment should they be left in contact with water for any length of time.
If an aquarium thermometer breaks, although it is not composed of any inherently toxic materials, any pieces should be removed to ensure the environment is not damaged over time.