Depending on what style of G&D Chiller you decide on, there are 2 different types of sight glasses on the reservoir. 1 is a window cut out on our polytanks. On our stainless steel tanks, there is an actual sight glass. That is a clear piece of tube that allows you to see the level in your system. Typically, you want to see your reservoir 2/3rds full at all times during the normal running process.