Protein skimmers are typically used in saltwater aquariums, as they are designed to remove dissolved organic compounds (such as proteins) from the water. These types of compounds can be a problem in saltwater aquariums because they can contribute to the buildup of harmful microorganisms, such as bacteria and algae.

Protein skimmers can technically work in freshwater tanks, but their efficiency and effectiveness will be greatly reduced. In freshwater aquariums, dissolved organic compounds are typically not much of an issue, so a protein skimmer may not be necessary. Some aquarists who keep heavily stocked or sensitive freshwater species may use a protein skimmer as a way to keep water conditions optimal, but it may not be as effective as in a saltwater tank.

It would be less common to use protein skimmer in freshwater tank because it's not as necessary, there are other ways to maintain good water quality such as mechanical, chemical and biological filtration.