Aquatics Unlimited: Articles: Thermometers
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for the week of 10/8/98

Since stable water temperature of an appropriate range is a life and death issue for tropical fish, it follows that a reliable thermometer is a vital component of an aquarium setup. Here are some features of the three common styles of aquarium thermometers.

Liquid-Filled Thermometers have been around for ages and consist of a thin glass tube partially filled with red liquid attached to some sort of scale. Three commonly seen varieties are "floating" (the tube is encased in thin glass and partially weighted to float upright), "standing" (more weight than floating to stand still on tank bottom) and "hanging" (thin tube is attached to bent piece of stainless steel or plastic to hang over tank's edge).
Advantages: these thermometers are quite inexpensive and their familiar design makes them quite readable for most aquarists. They generally work over a wide range of temperatures, can be easily moved from aquarium to aquarium, and react fairly quickly to change in temperature. They can be used to test tap water temperature before adding water to aquarium.
Disadvantages: they are not particularly accurate (check the variance on a dozen units hanging on the same hook at the store), but generally the aquarist can figure out how far off an individual thermometer reads and compensate. They are also easily broken, and the liquid and/or weights released have been blamed for tank problems. Finally, many of these thermometers have a printed or stamped "Safe Range" of ten or more degrees - far too wide a span for most fish.

Liquid Crystal Thermometers have become very popular in recent years, and are perhaps the most practical application of Mood Ring Technology, in which man-made crystals change color over a certain narrow temperature range. For example, one crystal might be black at any temperature below 76 degrees, orange at 76, green at 77,blue at 78, then black at any temperature over 78. A typical thermometer actually consists of a row of such crystals, each having a slightly higher reaction temperature than the one preceeding it, embedded in a very thin sheet of plastic, which is then adhered to the outside of the aquarium. Since there is some overlap in coverage, it is common for two, or even three, crystals to be colored at one time, which seems to confuse some aquarists.
Advantages: LC thermometers are inexpensive and easy to install, and generally very accurate and consistent from unit to unit. They are nearly unbreakable, and there is no risk of toxic elements being released into the aquarium.
Disadvantages: LC thermometers can be difficult to read in low light, and usually have a rather limited range (higher or lower temperatures result in all black crystals). Since they are attached to the outside of the aquarium, there is a remote possibility that a strong draft might give a false reading, but generally speaking, the aquarium glass (and the attached thermometer) will be the same temperature as the water in it, rather than the air surrounding it.

True Digital Thermometers are occassionally seen in the hobby and consist of a small probe that hangs into the aquarium and is attached to a small digital readout that is mounted outside the tank. Readings are often in tenths of a degree, which should be accurate enough for just about anybody, and some units can be programmed to sound an alarm if the temperature wanders out of a certain range.
Advantages: digital thermometers are very accurate, and very easy to read. Unless permanently mounted to the aquarium, they can be easily moved, and can be used to test water from the tap or in a holding/mixing bucket.
Disadvantages: these thermometers are much more expensive, and require batteries to operate (although batteries do last a very long time). The display meter is not submersible and can be dameged by water or rough handling.

"Tip of the week" appeared regularly in 1999 and 2000.

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