Aquarium Thermometers are a great way to measure the temperature of the fish tank easily. Knowing the temperature is vital. If the temperature fluctuates too much it can cause your fish to stress and get sick. There are a few types of thermometers like the sticker thermometers which stick onto the outside of the aquarium, glass thermometers which sit inside the aquarium or even electronic ones that use a probe to read the water and display the results on a easy to read LCD Display.