Okay, I know this sounds bizzare, but a meat thermometer is great for quick accurate readings of tank water temperature.
The type of meat thermometer has a round temperature dial mounted on 4" metal spike. Stick the spike all the way into the water and you'll get an accurate reading in seconds.
This tool is especially handy if your tank has a 'mood ring' outside-the-tank thermometer stuck to it, and the thermometer reading is being corrupted by over hot/cold air temperature.
I tested the one gallon tank several times last night, with the meat thermometer - as my Tetra heater was 'under the suspicions', as Inspector Clouseau used to say.
Miss America, my betta fry, swam right up to the meat thermometer, inspected it and bit it, lol! Nevertheless, the thermometer did its job.
The point here is, that no matter what the reading on the outside thermometer was, the inner temperature remained constant. The meat thermometer proved that the Tetra heater was truly working properly.
Hope you find this tip useful...