Yesterday I bought a new digital thermometer. The kind that hangs on the outside of the tank, and has a probe that you submerge in the tank.

Well it's reading 73F! This is confusing me, because my regular glass, mercury thermometer is reading 79F. Soooo which one is screwing up?? I mean if I go get a new heater and turn it up so that the digital reads 79, and I kill my fish because the thermometer was screwed up I'll be heart broken.

What's going on??