Marine (Saltwater) Aquarium Protein Skimmers

Information and opinions here are not only based on my experience in the Aquarium Maintenance business (over 28 years) but also of others I know in this business who have to use these devices on a regular basis.
(This is still an article in process, updated 1/14/08)

Basics
A protein skimmer is a filtration device used in marine tanks (they do not work in freshwater) which employ a chamber with a column of fine bubbles. Surface tension attracts organic waste to the bubbles & carries it through the column (called foam fractionation); then it is "skimmed" into a collection cup. This process Protein skimmers use to remove nitrogenous wastes (protein based organic waste) is also called foam refraction. The protein skimmer collects this waste in a cup, where it is then emptied (either by hand or a drain as in larger “Pro” models).
Protein Skimmers come in pump driven and air driven models. Within the pump driven there are different types: Venturi, Aspirating, Downdraft, and spray injection; the first two being most common.
Protein skimmers generally only work in marine aquariums where they are very popular in reef aquaria, as they are often needed to keep nitrates below .20 ppm for the delicate marine invertebrates.

The advantages are that they remove nitrogenous waste before they can go thru the nitrogen cycle and become nitrates. The disadvantages are some are messy, take frequent adjustments (at least on many commercial models), and in my experience, over rated especially for marine fish aquariums. I also have kept many reef aquariums successfully with and without protein skimmers although I do recommend protein skimmers for reef aquariums.
What I personally find interesting is that many of the aquarists that swear by protein skimmers totally trash UV Sterilizers, yet I have found from experience (I had to service what I sell, and if I sold trash, I had to service trash!) that often UV sterilizers had a more positive attributes than protein skimmers, although many of the newer models are vastly improved. That said, a good Protein skimmer is a useful tool in marine aquariums especially when used in conjunction with the Berlin Filter Method for reef aquaria and should be considered, more so if you are planning on keeping delicate corals!

Air Driven
The co-current air driven models (also called columnar skimmers) are the most basic although not always the simplest to use as these cheap Skimmers are often temperamental to water levels in the tank and the limewood air stones used by these skimmers clog frequently. However these can be an inexpensive entry level Skimmer for 10 -60 gallon marine aquariums.

Venturi
Of the pump driven Skimmers, the co-current Venturi models are the most common and popular. This style uses a water venturi pump (sometimes referred to as an aspirator) which is used to introduce the bubbles into the water stream. The tank water is pumped through the venturi, in which fine bubbles of the correct size and number are introduced then enters the skimmer body.

However not all venturi skimmers are equal, some of the more basic designs such as the Bak Pak work well, but the newer designs such as the V2 Skim by Tropic Marine also has a patented Bubble Stop in which the skimmer is fed 'dirty' water from the aquarium as a means to re-circulate water within the skimmer multiple times before it is returned to the sump or the aquarium.
The Tropic Marine V2 Skim is one of the better ones with the latest in technology. This skimmer has a venturi injection system which optimizes the perfect mixture of fine air bubbles and water and ensures intensive, efficient skimming and the removal of proteins and other harmful toxins (waste) from the aquarium. This Skimmer also has a Bubble stop feature that essentially re-skims the water as well as prevents bubbles/foam from re-entering the aquarium. For advanced Marine Aquarists (or even beginners looking to step up) this is the Protein skimmer I would strongly recommend. Each time water is re-circulated within the skimmer any air bubbles in that water sample are destroyed and new bubbles are generated by the re-circulating pump venturi apparatus so the air-water contact time begins again for these newly created bubbles.
For the money, this is the best Protein Skimmer you can buy unless you are willing to spend several hundred dollars more for the ASM skimmer.

Aspirating; Needle Wheel
An aspirating Protein Skimmer generally uses an impeller which consists of a mesh material that is attached to a plate or central axis on the rotor. The purpose of these modified impellers is to chop the air that is introduced via a venturi apparatus or external air pump into large amounts very fine bubbles. This style (such as used in a model by Coralife) is gaining popularity; however this style still has some bugs in that not all of the impellers used by these always chop the air bubbles properly due to anomalies in the flow and impeller design. Part of the problem with this style skimmer is that it does NOT lend itself to cheap low cost designs, so quite bluntly if you want an aspirating/needle wheel skimmer that works, be prepared to spend 100s of dollars (and in the case of the ASM Protein Skimmer they REALLY work)
The ASM Protein Skimmer is the ONE skimmer in this category that I can recommend based on the high end marine aquarists that use it. What differentiates this aspirating skimmer from the rest is that has a unique "custom" impeller housed in the specifically modified Power Head that chops the bubbles very finely. Another aspect of this aspirating skimmer is that it combines a venturi with its custom meshing needle wheel pump. The negative to the ASM Protein Skimmers is the price, as these units will retail over $700 + dollars.

Downdraft
A downdraft protein skimmer injects water under high pressure into tubes that have a foam or bubble generating mechanism then carry the air/water mixture down into the skimmer and into a separate chamber. This design is generally used in large aquarium systems uses tubes with plastic media (often bio balls) inside to mix water under high pressure and air in the body of the skimmer resulting in foam that collects protein waste in a large collection cup.

Spray Injection
The Spray injection skimmer ( a take off from the Downdraft Skimmers) uses a pump to power a spray nozzle, set a few inches above the water level. The spray action entraps and shreds the air into very fine bubbles in the base of the unit, which then rises to the collection chamber.

The Spray Injection Skimmer is a fancy new name for the Downdraft style Skimmers which work well in the large industrial size environment in which they were originally designed for, however this design does not lend itself to small sizes well.
This style skimmer also can clog with time; also one of the more popular models of this is sensitive to water levels and can be temperamental when used in a sump environment.
These skimmers are really being hyped and over promoted, but personally I would recommend steering clear of these units, not that do not work, however for the same price a V2 Skim will far out perform it and for far less money you could probably do just about as well with a Rio Nano Skimmer or a Via Aqua Multi Skimmer

Basic Skimmers, (Skimmer filter combinations)
A couple of basic skimmers for beginners; Via Aqua Multi Skimmer with small to medium aquariums is relatively simple, with a mechanical filter cartridge, bio filtration ability, and most of all the Multi Filter has a built in UV Sterilizer combination.

For a really simple protein skimmer for a Nano tank I recommend the Rio Nano Skimmer.

Conclusion

Here are some important aspects to consider so as to a purchase a skimmer that performs correctly:
• Contact time of bubbles and water inside the chamber (the longer the reaction chamber, the better in most cases)
• The size and consistency of the bubbles produced (very fine bubbles are best)
• The relative volume of bubbles produced
• The ratio of air to water
• And the turbulence or friction inside the reaction chamber which may cause poor efficiency in some poorly designed units.

Please beware of claims of some high priced skimmers such as Spray Injection models as these units do not have the contact time (due to short/small reaction chamber) to maximize protein refraction. Although units such as these due have excellent size and volume of bubbles, the small size of the reaction chamber minimizes effectiveness so that one of these units may perform better than similar size entry level skimmers such as the air driven Lee’s or the Rio Nano/Via Aqua Multi Skimmers venture skimmers; the price at nearly $200 is nowhere near the increase in effectiveness over a Rio Nano Skimmer (or similar) that costs under $30 as a price vs. increase in effectiveness comparison.