Place on the tank glass outside the aquarium tank Suitable for cold water, tropical and marine aquarium. If the surrounding temperature is 27°C, then the background of scale mark 27°C will turn colorful and other scale marks remain black. Easy to install, just peel off the back of the sticker and the transparent protective film at the front and stick it on the outside wall of the aquarium, that is it. Simple to use and convenient to read. Unlike traditional thermometer that takes forever to figure out where that little red bar in the tube sits.