JBL Mini Glass Aquarium Thermometer

JBL Mini Glass Aquarium Thermometer is a small alcohol-filled glass thermometer that suction-cups inside the aquarium.

Key Features

• No battery, so nothing fails quietly: A glass thermometer either reads or is visibly broken. A digital one can sit there showing yesterday's number on a dying battery, which is exactly why most experienced keepers run a glass thermometer alongside whatever else they use.
• Alcohol filled rather than mercury: The blue column is non-toxic alcohol. If it ever breaks in the tank it is a nuisance to clear up rather than a disaster.
• Small, and it does not have to go on the front glass: Suction cup mounted, and JBL note it also clips onto a CristalProfi m internal filter, so it can be tucked out of the sightline on a display tank.
• Scale from 0 to 50 degrees: Marked in two degree steps, which makes it a clear check reading rather than a precise measurement. That is exactly what a backup thermometer needs to be.

Why Any Tank Needs One

• A heater tells you what it is trying to do. A thermometer tells you what is actually happening, and those two are not the same thing once a thermostat starts to drift.
• Heaters generally fail stuck on rather than stuck off, and a tank being slowly cooked gives very few other warnings until fish start gasping at the surface.
• Temperature is also the first thing to check when anything else goes wrong. Fish off their food, fish scratching, an outbreak of whitespot: all of them point at the temperature before they point anywhere else.
• Read it at the opposite end of the tank from the heater. A reading taken next to the element tells you about the element rather than about the tank.
• Keep two if you can, one digital and one glass. They cost very little together and they catch each other's errors, which a single thermometer by definition cannot.

Using It

• Site it at the opposite end of the tank from the heater, low down and away from the filter outlet.
• For setting an exact temperature use a digital thermometer and keep this as the second opinion. Together they cost very little and they disagree loudly when one of them is wrong.
• Give it a few minutes after moving it before you trust the reading.
• Check the suction cup from time to time. Cups perish in warm water, and this is glass, so a cup that lets go drops it onto the substrate.
• Keep it where a large or boisterous fish cannot knock it off the glass.

Good To Know

• Glass. Handle it carefully, keep it away from children, and replace it if the column separates or the glass is chipped.
• The two degree gradation is a check scale. If you need to see a half degree of drift, that is what a digital thermometer is for.
• Rinse it in tank water rather than tap water when you clean it, and never use detergent on anything that goes in the tank.
• At this price it is worth having one in every tank, including quarantine and fry tanks where temperature matters most and equipment usually matters least.