There are hosts of problems with cheap glass thermometers, from miscalibration (scale is on paper, which isn't always aligned or properly attached), to separation of the fluid caused by shaking or overheating, which will give permanently bad readings, to bad manufacturing, to breakage, leaving sharp shards of invisible glass to cut you later.
I don't trust them, and don't have them in either of my display tanks.
The corallife digital is only $15 at Big Al's, and those I own are right on (agree on the temp, and agree with a medical thermometer which I do trust).
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