Protein Skimmer

What Is A Protein Skimmer?

Protein skimming, or foam fractionation, removes dissolved organic compounds (DOCs) from the aquarium water before it gets a chance to break down into ammonia.

How A Protein Skimmer Works

The pump within the protein skimmer generate millions of tiny air bubbles.   Dissolved organics in the water adhere to the surfaces of these air bubbles and foam is created.  The foam builds and gradually rises up the neck of the skimmer, overflowing into a collection cup where it can be discarded.

Very early skimmers — known as counter current skimmers — used an air pump and a limewood airstone to generate these bubbles and were inefficient compared to modern protein skimmers.  Contact time between the air bubbles and the dissolved organics in the water was minimal, resulting in weak, watery skimmate.  Or no skimmate at all!

In modern venturi skimmers, water is injected through a valve at great velocity to produce bubbles.  Some designs use an air diffuser, pulling in surrounding air to create bubbles.  Either way, the resulting water and air bubble mix then enter a reaction chamber where contact time between the bubbles and dissolved organics are maximized.  The resulting foam, now heavily laden with organics rises up into the collection cup.

protein skimmers by Skimz

Cone protein skimmers by Skimz

Protein Skimming And Trace Elements

Protein skimmers are usually run 24/7 but since they also skim off trace elements, some aquarists prefer to run them part of the time, choosing to rely on other more natural forms of filtration like algae/mud systems and refugiums.

But running a protein skimmer continuously is definitely advantageous for the health of the aquarium especially if we are not able to have a sump or refugium as part of our system.  Any depletion of trace elements can simply be replaced by regular 15% monthly water changes or by dosing these elements with additives.

The Difference Is In The Foam

A good protein skimmer, running on a well-stocked tank full of fish should be able to produce a thick, dry foam or skimmate.  When the foam collects in the cup and collapses back into a liquid, the resulting skimmate should be a dark greenish-brown color.

But it should be mentioned that skimmate production will not be as impressive in a tank that is lightly stocked.  Tanks that are filled mostly with coral and only a few fish will usually not yield much skimmate, unless feeding is particularly heavy.  A light, tea-colored foam can be routinely be expected from such tanks, no matter how efficient the skimmer.

Protein skimmers can be expected not to skim for a few hours after a feeding, especially of omega-3 rich oily foods like Cyclopeeze.  This is because the oil in the food breaks the surface tension of the water, reducing the production of bubbles, collapsing the foam column in the neck of the skimmer.  Foam production should resume in several hours.

Needle Wheel Protein Skimmers

Some manufacturers modify their protein skimmer pumps with a needle wheel.  As air is drawn into the skimmer, the needle wheel chops up the water to produce even finer bubbles than ever before.  The resulting foam-like consistency of the bubbles ensures copious amounts of skimmate.  Needle wheel equipped protein skimmers have become the gold-standard for efficient skimming.

Protein Skimmer Sizes

Manufacturers generally indicate a recommended tank size for each of their skimmer models.  Buying a severely underpowered protein skimmer, relative to your tank size, is not a good idea . The skimmer will not be able to handle the bioload of your tank. But buying a skimmer that is a size above the recommended model for your tank is always a good option.

Better to overskim than to underskim..


Protein Skimmer Placement

There are basically three types of protein skimmer — Hang-on skimmers, In-Sump skimmers and Out-Of-Sump skimmers.

protein skimmer

Large protein skimmer placed in-sump

In-Sump Protein Skimmers

An in-sump protein skimmer will, of course, be ideal if you have a sump.  The protein skimmer is a large piece of equipment and placing it in the sump puts it out of view, and away from your main display tank.

It is very important that the in-sump protein skimmer be placed in a section of the sump where water flow is not turbulent.  Churning water will dramatically reduce the efficiency of the skimmer.  Some in-sump skimmers made by Tunze can also be made to work as a hang-on skimmer with the simple attachment bracket provided.

An in-sump protein skimmer should always be placed before the refugium as well as before any other filtration media such as activated carbon, Polyfilter or phosphate adsorbing media such as Rowaphos.

Out-Of-Sump Protein Skimmers

If space is not an issue, and if the skimmer is designed as such, an in-sump skimmer can also be run outside the sump.  This simplifies routine cleaning and maintenance as the skimmer can be thoroughly cleaned without having to first remove it from the sump. Running a protein skimmer out-of-sump requires an extra pump to supply water to the skimmer.

Extra large skimmers are designed exclusively for out-of-sump use.  The protein skimmer pictured at right is about 3 feet tall.

protein skimmer

Large out-of-sump protein skimmer

Hang-On Protein Skimmers

A hang-on protein skimmer is usually designed for smaller tanks without a sump.  A common problem with hang-on skimmers is the amount of fine air bubbles that are inadvertently returned to the main display tank.  Placing a piece of sponge or filter floss immediately where the water from the protein skimmer discharges back into the main tank will help alleviate this problem.  The sponge or floss should be rinsed out weekly to prevent detritus buildup and potential nitrate problems, especially in a small tank where nutrients build up quickly.


If you have a tank between 20 – 75 gallons, AquaC’s Remora protein skimmer models are excellent.

 Tunze Protein Skimmer Not Skimming Enough?

My experience with the Tunze 9010 and 9021 skimmers is interesting.

Tunze 9010 skimmate

Skimmate from Tunze 9010

The Tunze protein skimmer generally produces lighter, tea-colored skimmate in a tank that is moderately stocked, leading many to think that they are inefficient.

But upon closer investigation, we will find that where Tunze skimmers lack in producing dark, wet skimmate in the collection cup, they more than make up for in the sheer amount of thick gunk that accumulates in the neck of the skimmer after just 3 or 4 days!

Surely thicker and more concentrated than any amount of dark liquid skimmate and worth the price of admission alone!


The Tunze protein skimmer works extremely well, maybe just not in the way we would like to see it.

Cone Protein Skimmer Designs

The cone skimmer is a relatively new design innovation in the world of protein skimmers.  The body of the skimmer is shaped into cone cylinder which maximizes bubble consistency while maximizing contact time of the bubbles and dissolved organics.  The cone skimmer can therefore be built smaller and shorter while remaining efficient.  Similarly, the pump size and wattage can also be reduced since less water pressure is required to generate the same amount of foam.

If you haven’t already purchased a  protein skimmer, or if you’re looking to upgrade, a cone skimmer is definitely worth considering for its efficiency.

Read more about:

Tunze 9012 DC Skimmer Product Review

Protein Skimmer Tuning

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