Do Fish Tank Thermometers Contain Mercury?

Do Fish Tank Thermometers Contain Mercury?

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So, you’re looking at your fish tank and you notice something strange. You realize one of your fish broke the glass on the thermometer. What’s more, the thermometer is leaching its contents into the water. You have to remove the broken parts as soon as possible.

But this begs the question, do fish tank thermometers contain mercury? Some thermometers do contain mercury. However, there are others that can contain alcohol, kerosene or galinstan. These latter three don’t present the same toxicity as mercury. Knowing how to handle and clean any of these substances will be imperative.

Do fish tank thermometers contain mercury?

There are some tank thermometers that do contain mercury and these give the most accurate readings. You’ll know it by the appearance of the grayish, silvery metal substance resting in the thermometer’s inner tube.

When the glass breaks, small spheres bleed into the tank, presenting the highest risk of lethality. This is true for you, your fish, plants and any other life that exists in the tank. The good news is, because they are so toxic, there’s been a ban on mercury thermometers since January of 2003.

How do you remove mercury when it leaches into the aquarium?

In the unfortunate instance that mercury contaminates your aquarium, you have a small window of time before it begins killing your plants and fish. Follow the steps below to take care of the problem right away:

  1. Put on a pair of thick, sturdy gloves ( Rubber, nitrile, or latex gloves ) to avoid coming into contact with mercury.
  2. Capture all the fish as fast as you can and put them into clean water.
  3. With a pipette or other narrow-needle-type syringe, suck up the mercury spheres floating around.
  4. Empty out all the water from the tank.
  5. Rinse off any plants and the substrate thoroughly.
  6. Put the plants in with your fish and allow substrate and the tank to dry. This is because mercury evaporates at room temperature.
  7. Then put the plants and substrate back into the tank.
  8. Fill the tank with fresh, clean water and return the fish to their home.

Are there any other substances contained within thermometers for fish tanks?

Since mercury has been illegal for thermometers since January of 2003, there are several other things manufacturers use. These aren’t as accurate as mercury, but they are somewhat safer.


When the thermometer has a red color sitting at the bottom of the tube, it’s more than likely alcohol. The metal-colored balls producing the red hue is what makes this thermometer heavier. If it breaks, a small amount of alcohol releases into the tank. This isn’t nearly as harmful to fish and plants as mercury.

You won’t have to do a whole sweep and clear of your system either. But, you will have to remove the glass and metal balls to prevent injury to the fish. Some aquarists report that some fish do try to eat these little metal balls, mistaking it for food. Leave the cover open for about 30 minutes so the alcohol evaporates.

Petroleum-Based Kerosene

Some types of thermometers contain a petroleum byproduct like kerosene. When this gets into the water, it will fall to the bottom of the tank. But, with running equipment, this will eventually begin floating around.

Kerosene doesn’t present an immediate problem to the fish and plants in your tank if it breaks. The substance will float to the bottom at first. However, eventually, it will begin to float around the tank and it can become a detrimental issue.

Therefore, you will have to clean the tank right away because it can affect water quality in profound ways. If it gets into your fish’s gills, it can cause them to suffocate. 

  1. Turn off all equipment and only put the fish into clean water if the kerosene is all over the tank.
  2. Pump out the water where substrates, plants and decorations came into contact with the kerosene.
  3. In the case the kerosene scatters and floats throughout the tank, use an oil-absorbing paper like facial tissue, toilet paper and etc. Drag the paper across the affected area in the water and pull it up.
  4. Perform a water change if you think the problem was bad enough.
  5. Leave the tank’s cover open to allow any residual kerosene to evaporate.


Galinstan is another substance manufacturers use to make tank thermometers. This is an alloy of tin, gallium and indium, producing a silver/gray liquid similar in appearance as mercury. But, galinstan isn’t toxic. That said, you will have to clean up the mess if it breaks inside the tank.


Although fish tank thermometers can contain mercury, there are many other substances they can comprise too. For any thermometers produced after January 2003 and you see a grayish silvery material in the tube, chances are it’s galinstan.  

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