A protein skimmer, also known as a foam fractionator, is a device used in marine aquaria, and particularly in saltwater aquaria, to remove dissolved organic compounds (DOCs) from the water.

Protein skimmers work by creating small bubbles which are introduced into the water. As these bubbles rise through the water, they come into contact with dissolved organics, and the organic molecules attach to the surface of the bubbles. As the bubbles rise to the top of the skimmer, they become trapped in a collection cup, and the organics that have attached to the bubbles are removed from the water.

There are several different types of protein skimmers, but the most common types are air-driven and venturi-driven skimmers.

The air-driven skimmers are the simplest and most basic type of skimmer. They use an air pump to create a stream of bubbles, which rise through the water in the skimmer.

Venturi-driven skimmers, on the other hand, use a venturi injector to create a high-speed stream of bubbles. This stream of bubbles is introduced into the water in the skimmer, and as the bubbles rise through the water, they create a turbulent flow that helps to strip the dissolved organics from the water.

Protein skimmers are considered essential equipment for a saltwater aquarium because they help to keep the water clean and clear, and they remove dissolved organics that can contribute to the growth of undesirable microorganisms like bacteria, algae and also lower the pH levels. They also have the ability to decrease the load on filtration and biological system, which ensures a healthy and stress-free environment for the inhabitants of the aquarium.