A thermometer is an instrument for measuring temperature, especially one having a graduated glass tube with a bulb containing a liquid, typically mercury or colored alcohol that expands and rises in the tube as the temperature increases. Choosing a good quality, reliable thermometer can save you lots of money in the long run. Since saltwater animals are extremely temperature sensitive, knowing the temperature in your tank is very important. There is really no "ideal" temperature which you should keep your aquarium at. Every hobbyist has their own opinion and different animals from different locations require various temperatures. An optimal range should be anywhere from 73° to 80°, it all depends on what type of aquaria you keep in your setup.