I have a bunch of tanks, and would need a truckload of mercury thermometres, most of which would give the wrong temp anyway. I can remember taking a good guess at the temperature in a store, then seeing which thermometres were close. There was that trick of seeing if you could find three of them with the same temp, since those ones might be accurate. We had some weak tech in this hobby.
They were more reliable than putting your elbow in the water.
I have some old clip on heaters in a drawer somewhere. It's a wonder we didn't all die from electrocution. The little plastic screws that held them on to the rim are flimsy.
Tech used for the good of fishkeeping humanity is fine in my books, although my paper books are obsolete...