A protein skimmer, also known as a foam fractionator, is a device used in marine aquariums to remove organic compounds, such as proteins, from the water. These compounds are produced by fish waste, uneaten food, and other organic matter in the tank.

The skimmer works by creating a column of bubbles that attract and trap the organic molecules. The bubbles rise to the surface of the water and form a foam, which is then collected in a collection cup and removed from the tank.

There are several types of protein skimmers available on the market, including:

1. Air-driven skimmers - These use an air pump to generate bubbles, which rise up through a column of water and create the foam.
2. Pump-driven skimmers - These use a water pump to create a stream of water that is forced through a venturi valve, which produces the bubbles.
3. Venturi skimmers - These use a venturi valve to create a vacuum that pulls in air, which creates the bubbles.
4. Recirculating skimmers - These use a pump to recirculate the water through the skimmer, which allows for more efficient removal of organic matter.

In a reef tank, a protein skimmer is an essential piece of equipment for maintaining water quality. By removing organic compounds before they break down and contribute to high levels of nitrate and phosphate, skimmers help prevent the growth of algae and other harmful organisms.

When choosing a protein skimmer for a reef tank, it's important to consider the size of the tank, the bioload (number and size of fish and other inhabitants), and the type of skimmer that will work best for your setup. It's also important to properly maintain the skimmer by cleaning it regularly and ensuring that it is functioning properly.