Monitoring the temperature of your aquarium is vital for the health of your fish. Sudden temperatures changes can stress the fish out. Use an aquarium thermometer to measure the temperature of the fish tank. Its as easy as sticking a sticker thermometer onto the front glass of the aquarium. There are also many types of thermometers like the popular floating glass thermometer, digital thermometers that uses a probe to measure the temperature of the tank and displays it LCD screen outside the tank.