Hobbyists use various types of protein skimmers for marine aquariums. Each type of skimmer has several pros and cons. Different designs vary in cost and efficiency. Which protein skimmer setup is best depends on your specific concerns and requirements.
Protein skimmers use a process called foam fractionation to remove organic waste from aquarium water. Such devices consist of a main tube and a pump that drives aquarium water through the device. Within the skimmer, tiny bubbles are generated. The bubbles float through the main body of the skimmer, collecting proteins and other organic waste with their surface tension. The bubbles collect in the top of the skimmer as foam, which you'll discard. The main difference between the types of aquarium skimmers mostly involves the way they generate their bubbles. The smaller the bubbles, the more efficient the skimmer and the shorter the skimmer body can be.
Venturi skimmers take advantage of certain principals of fluid physics to generate extremely tiny bubbles. Using a principal called the venturi effect and special valves, venturi pumps produce the finest foam of any protein skimmer. This allows them to have short bodies, making them compact. Additionally, these skimmers typically require very little in the way of adjustments and fine-tuning, making them "set and forget" skimmers.
Airstone skimmers generate their bubbles with an air pump and a piece of porous hard material called an air stone. The foam they produce is not as fine as that of other protein skimmers, so they tend to have larger bodies to compensate. On top of this, air stones require periodic replacement. They also require adjustment to get the foam right. Despite their limitations, their low cost keep them in the game.
The Best Skimmer for You
When selecting a protein skimmer, the cost and the amount of space you have will probably make the decision for you. If you have a limited budget, an air stone makes a fine skimmer. It will require more space and more of your time. However, if you have limited space -- like a sump in an aquarium cabinet -- but you have the funds, a venturi skimmer will be the best option for your setup. Your priorities and your resources, in the end, determine which protein skimmer is best for your aquarium.