Ha, Ha. Although I'm sure that method of testing aquarium thermometers works, it sounds like a practical joke some older fish keeper would play on a novice. Maybe some people are more adventurous than me, but I would never put anything from an aquarium under my tongue. I wouldn't do it even if the item had been triple washed, soaked in strong bleach water for an hour, soaked in 70% alcohol for an hour, left to dry for a week, and then soaked in alcohol again.
For other risk-adverse readers, a less risky alternative might be to wash the thermometer off and then seal it in a ziplock bag being sure to squeeze all the air out and then place it in your bare arm pit then lower your arm and hold there for two minutes. Nurses used to sometimes test body temperature in the arm pit this way. It might not be as accurate as other body sites, but it is probably close enough to test an aquarium thermometer.
