A pond thermometer is required to keep your pond at the right temperature for your fish. Pond thermometers come with a hole or ring attached to one end so it can be hung from a string, hook or rope, or suspended by a tether cord. For the most adequate temperature readings your pond thermometer should be taken at the half way point in depth of the water. Pond thermometers are necessary if you move your tropical pond plants or pond fish indoors during the winter months. Tropical pond plants should never be exposed to temperatures below 40 degrees Fahrenheit; while pond fish should be moved indoors before the pond water temperature dips below 55 degrees Fahrenheit. Pond thermometers have both Fahrenheit and Celsius scales. To read the temperature, simply pull the thermometer out of the water. Pond thermometers usually go for around $10.