You can see here that how a digital thermometer is cheap and sofisticate to use for fish tank aquarium.
My both thermometer i.e. Glass thermometer and Digital thermometer shows same temperature.
I have also tested the digital thermometer in two conditions as below,
1) hold the digital thermometer sensor 2.5 feet over the flame of cooking stove.
Temperature – 40.9°C
2) In the freezer for 5 minutes.
Temperature – 0.9°C
It means that it works properly.