I use a 3" dial thermometer in my tun. It's mounted fairly low, maybe 3" from the bottom of the tun and lines up with the bazooka filter.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk