Which thermometer should I trust?

[tcarswell]

You could always buy a cheapo glass thermometer for 2 bucks. Not super accurate but it should be able to tell you which thermometer is true.

 

[bob965]

It sounds like you're calling the dial setting on your heater a thermometer. That's just a thermostat, that will give you a good ballpark of what the temperature will be, but you still need to check it with a separate thermometer. It sounds like you're using a thermometer with an LED display and remote probe. If that says 72.5, then your water is probably 72.5 degrees as they are usually pretty accurate. Please excuse me if I'm misunderstanding you.