Broken aquarium thermometer

Help a fish broke my aquarium thermometer – Don’t panic!

An aquarium thermometer is used to keep an eye on the temperature of the aquarium. They are usually made out of glass which contains a liquid that indicates the temperature. But what should you do if your aquarium thermometer is broken and the content has ended up in your aquarium? First of all: Don’t panic! Most thermometers used in aquariums do not contain any harmful substances!

To give you an idea of the types of thermometers, we list them below, from not dangerous to (for the fish) deadly mercury thermometers.

Types of aquarium thermometers

Aquarium thermometer met alcohol - herkenbaar aan de rode kleurstof.
Aquarium thermometer with alcohol – recognizable by the red color.

Alcohol or Petroleum thermometer: This thermometer can be recognized by the red liquid in the column and the ball at the bottom. The red color is a colorant that has been added to the alcohol or petroleum. To make the bottom of the aquarium thermometer heavier, metal-colored balls are often added. If this thermometer breaks, a very small amount of alcohol is released into the aquarium. Alcohol in this amount is not harmful to the fish or plants. Of course you have to remove the sharp glass and the metal balls to prevent fish from injuring themselves or swallowing them.

Galinstan thermometer: This thermometer contains an alloy of gallium, indium and tin. The liquid is silver / gray. It is a metal alloy that is liquid at room temperature. Just like mercury thermometers, they are very accurate. If a thermometer with Galinstan breaks, the liquid metal does not form spheres. Galinstan itself is not directly toxic to fish and plants. Of course, just like with the alcohol thermometer, it must be cleaned up as soon as possible. I personally have never seen an aquarium thermometer that uses Galinstan.

Kwik thermometer - herkenbaar aan de metalen/grijze inhoud onderin de thermometer
Mercury thermometer – recognizable by the metal / gray content at the bottom tip of the thermometer

Mercury thermometer: The mercury thermometer is recognizable as the silver / gray liquid contained in the inner tube and the globule. When the mercury flows from the tube through a fracture, small spheres form. The Mercury thermometer was invented in 1714 by Gabriel Fahrenheit. He also immediately came up with a scale: Fahrenheit. The big advantage of a mercury thermometer is the accuracy, they are accurate to 0.2 degree. Mercury is very toxic and therefore no longer allowed in thermometers from 1-1-2003. If mercury is released into the water, the fish will die from mercury poisoning. Catch all your fish as quickly as possible and place them in clean (tap) water. Empty the entire aquarium. As a precaution, replace all plants and substrate before refitting the aquarium.

Mercury globules are very difficult to catch. The easiest way to do this is to use a pipette or syringe with a needle that sucks up the globule. Note Mercury evaporates at room temperature. Mercury vapors are toxic if you inhale enough. The amount of mercury in a thermometer is fortunately limited and will not quickly assume harmful concentrations.


Most aquarium thermometers are harmless to humans and animals. Long before the ban on the use of mercury came into force, many thermometers were already provided with a different content. The chance that you still have a mercury thermometer is not very big.

Advice: if you have a glass thermometer, view the inner column. If this column has a metal / gray color, it is possible that it is mercury. Because it must also be an old thermometer, the glass will slowly become a bit brittle. The chance of a break then becomes greater. Don’t just throw them away but treat them as chemical waste!

Copyright images

Michelle Roodbergen
Mercury thermometer by Jurii – CC BY 4.0 – Based on a work on



    Yup, that picture at the top is my thermometer.
    I’ve been doing aquariums since 1994 and have never had a thermometer break…in the tank with my brand new Dumbo betta. (not that it’s ok in ANY tank)
    I knew it wasn’t mercury, that was easy. It’s what that weight balls are made of that scared me.

  2. Hi, is the top picture of the broken thermometer a mercury thermometer? That’s what mind looks like and I’m concerned. The picture that say is dat erg?

    1. Hi, the top picture is an alcohol thermometer. So, no worries, you have a harmless one.

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