Most tropical fish have specific temperature requirements in order to thrive. The Penn-Plax AquaLIFE Therma-Temp Digital Aquarium Thermometer makes it easy to know exactly what temperature your aquarium is at. This thermometer mounts conveniently on the outside wall of your aquarium with a pressure-sensitive backing. The numbers are clear and easy to read as the green color shows the correct temperature. 2" strip Great for smaller aquariums Easy to attach Numbers are clear and easy to read Mounts on the outside of all glass aquariums INSTRUCTIONS: Green color shows the correct temperature of the water.