Glass thermometer (esp. the cheap ones) are not all accurate!! If you go look at the stack of thermometer at Wally's, they all read a different temp at the store, even when sitting at the same bin ....
You would want to use a high quality thermometer to take the temp before you junk your heater. A lab grade glass thermometer is of course the gold standard, but a good kitchen thermometer works as well. I use one of the instant read meat thermometer that can actually read down to 70. It is like $20, but gives accurate readings.
Depending on how the thermometer is made, you can calibrate it yourself. The standard lab calibration is: put in a cup of water with ice <let it equilibrate so there is both liquid water & ice>, and it should read 0C or 32F. Boiling water at sealevel should read 100C or 210F. <A bit less at altitude, you can find correction tables if you are up high.> You can then divide the 2 points into even intervals to find any temp accurately. Some thermometer don't have enough range to do this, then you have to calibrate to a known accurate thermometer. Most kitchen thermometer (even cheap ones) do have enough range for calibration, so is a better buy for your tank.
80 gal FW with 30 gal DIY wet/dry/sump.
9 fancy golds, 1 hillstream loaches, 1 rubber-lip pleco (C. thomasi), 3 SAEs, small school of white cloud minnows, planted.