It is rather ironic that one of the least expensive pieces of saltwater aquarium equipment (a thermometer) can save you the most money in the long run. Since tropical saltwater critters are extremely temperature sensitive, knowing tank temperature is very important. Aquarium thermometers come in many styles, the newest of which are the "Remote Sensor Digital Temperature/Time/Alarm" models.
About Stick-On Digital Thermometers
Digital thermometers which attach to the exterior of the aquarium are a popular, inexpensive and easy-to-use method of determining your tank temperature at a glance. Since they are always in the same place once you mount the thermometer, you don't have to "find the thermometer" to tell the temperature like you do with floating or bottom thermometers.
About Floating Thermometers
Floating thermometers can be attached on the inside of the tank with a simple suction cup mount or allowed to float around the tank. Constructed of plastic, these thermometers are more difficult to read than the digitals due to the short visible temperature range and their difficulty to see if they are free floating.
About Stainless Steel Thermometers
Hung from the inside edge of the aquarium with a stainless steel mount, these traditional "bubble type" thermometers are durable, easy to read and easy to locate in the tank.
About Standing Thermometers
Standing thermometers are units which are weighted to stay on the bottom as opposed to free floating on the water's surface. When used in tanks with wave makers, they can be difficult to find and read.
About Floating Thermometer or Hydrometers
These dual purpose thermometers float on the water's surface indicating the tank water temperature as well as the SG (Specific Gravity or salinity).
About Multi-Function Remote Digital Sensor Units
These multi-functional temperature, time, and alarm type units are among the most convenient to use. Simply hang the plastic probe in your tank and mount the digital display in a convenient place (wall or tank cabinet) up to 3' away from the tank. Some units even have an optional high/low temperature alarm.
