How do they work?
Skimmers rely on a useful property, where bubbles in salt water slowly become coated in protein wastes.
The very first skimmers were little more than a plastic tube, with a wooden air stone at its base. Bubbles produced from the air stone rose through the unit, becoming coated with proteins and pulling water through the unit in the process. The coated bubbles were then collected in a cup at the top of the unit.
Although better than nothing, the early skimmers were not very efficient at stripping protein waste from the water. Largely due to the relatively small number of bubbles produced being largely uncoated by the time they reached the collection cup.
Technology has since come to our rescue as many companies have invested their research budgets in producing ever more efficient beasts.
So what are the main things I should look for in a protein skimmer?
The ability to produce large quantities of small bubbles
Smaller bubbles have a larger surface area for a given volume of air, therefore removing more protein waste.
Older Technology: Wooden Air stone, as found on the first generation of skimmers. Largely replaced by venturi valve skimmers that use water flow produced by the pump to suck relatively large volumes of air into the skimmer unit.
Newer Technology: Needle wheel skimmers use a combined approach; a venturi valve to suck large amounts of air through the pump, where a specialized impeller chops the incoming air into very small bubbles to maximize protein uptake.
A long contact time between bubble and water
The ability to hold the bubbles within the reaction chamber long enough for them to become fully coated with proteins.
Old Technology: Co-current, bubbles and water flow straight through the reaction chamber with a relatively short time available for the bubbles to pick up protein waste. After this they developed counter-current technology, where the bubbles are forced to flow counter to the flow of water, increasing the time taken for them to exit the reaction chamber.
Newer Technology: Modern skimmers have taken counter flow technology to the next level by holding the air bubbles in a maelstrom of randomly swirling water. This maximizes the bubbles contact with the water in the reaction chamber, vastly increasing the efficiency.
A practical collection ‘cup’ design
The collection cup sits at the top of the reaction chamber and removes the coated bubbles from the skimmer, hopefully as a dry protein rich foam, that turns into a dark brown liquid.
Technology: The collection cups often use a shaped funnel to concentrate the foam into the cup, whilst allowing the foam to dry further by allowing water to drain back into the reaction chamber.
Personal preference: My suggestions would be to look for a skimmer with a decent sized cup to minimize the emptying required, and to ensure that the throat of the collection cup is not so narrow as to become rapidly clogged with waste in normal use. (Small throats can be a problem with some hang on models)