Your protein skimmer’s skimmate, the gunk that is removed from the water and deposited into the collection cup, can tell you a lot about what’s going on in your aquarium based on its color, texture, and even the rate it is collected. During normal operation, your skimmer will perform in a reliable fashion and the resulting sludge will be quite consistent from day to day. In my setups, the cup fills in about a week with a dark green liquid with brown sludge deposited in the neck of the collection cup. When things in the aquarium go awry, however, the skimmate will tell the story. Going back to my example of my own skimmer, if things are wonky in my aquarium, the production of the skimmate will increase and it will smell worse than normal. It can also be lighter in color and have fewer chunks of debris floating around in the collection cup. While this exact occurrence may not happen for everyone, it’s a safe bet to say that the skimmer will perform differently when there’s something wrong in the aquarium…so pay attention to its performance.