Protein Skimmer Basics for Marine Aquariums

How a protein skimmer uses foam fractionation to strip dissolved organic waste before it breaks down, why it only works in salt water, what it removes, and how to size and tune it.

Foam fractionation: how a skimmer works

A protein skimmer, or foam fractionator, injects a stream of very fine air bubbles into a column of water. Many dissolved organic molecules are amphipathic — they have both a water-loving and a water-repelling end — so they collect on the surface of the bubbles. As the bubbles rise they build into a dense foam that overflows into a collection cup as 'skimmate'. Because smaller bubbles pack far more surface area into the same volume, fine-bubble skimmers are more effective. The key benefit is that this exports dissolved organic waste before it breaks down and adds to the ammonia, nitrate and phosphate load.

• Proteins and amino acids from food and animal waste
• Fats, fatty acids and carbohydrates
• Dissolved organic carbon (DOC) released by fish and other animals
• Phytoplankton, bacteria and fine detritus
• Metals such as copper and trace elements such as iodine that adhere to the organics

A marine-only tool, and how to size and tune it

Protein skimmers are used almost exclusively on marine and reef aquariums. Salt water's ionic composition and surface tension let the fine bubbles form the stable, dense foam that skimming depends on; freshwater generally will not sustain that foam, which is why skimmers are a saltwater tool and do little in freshwater tanks.

Choose a skimmer rated for your display volume and its bioload. Since larger skimmers and longer bubble-to-water contact time both improve performance, it is safer to oversize than undersize. You tune a skimmer by adjusting the water level and air intake to set how wet the foam is: a wet skim overflows more, thinner skimmate and exports waste quickly, while a dry skim yields less, darker, more concentrated waste. Heavier stocking and feeding call for more aggressive skimming.

Sources: en.wikipedia.org , reefbuilders.com , en.wikipedia.org (CC BY-SA 4.0)