Protein skimmers are a key part of many successful reef aquariums, helping to remove dissolved organic waste. But how do they work, what makes a good skimmer, and what should you look for when choosing one?
If you've kept freshwater fish before, you may be familiar with mechanical filtration, but marine aquariums have another useful piece of filtration equipment: the protein skimmer.
Protein skimmers are a key part of filtration on many reef tanks, helping to remove dissolved organic waste before it can break down and pollute the water. They also help with gas exchange in the aquarium, making them particularly useful in heavily stocked reef systems.
But how do they actually work, and what should you look for when choosing one?
More from our Beginner's Reef Guide:
What is a protein skimmer?
A protein skimmer is a type of filtration equipment that uses tiny air bubbles to remove dissolved organic waste from aquarium water.
Seawater has different properties to freshwater, and dissolved organic compounds can attach to the surface of very small bubbles. A protein skimmer takes advantage of this by mixing aquarium water with a mass of fine air bubbles.
As the bubbles rise through the skimmer, dissolved organic waste sticks to their surface. The bubbles then rise into the collection cup at the top of the skimmer, carrying the waste out of the aquarium water with them.
The result is a concentrated, dirty liquid that you simply empty from the collection cup.
Protein skimming therefore provides an additional way of removing waste from a reef aquarium, helping to maintain the clean, stable water conditions that fish and corals need.
Related article: Back to saltwater: starting simple with beginner corals.
Why do reef tanks need protein skimmers?
Fish and other reef inhabitants constantly add waste to the water. Uneaten food, fish faeces and dissolved organic compounds can all contribute to declining water quality if they are allowed to accumulate.
Mechanical filtration can remove larger particles, while biological filtration converts toxic ammonia into nitrite and nitrate. A protein skimmer works slightly differently by removing some of the dissolved organic wastes before it has a chance to break down further.
This can help reduce the overall nutrient load on the aquarium and make it easier to maintain good water quality.
Healthy coral reefs exist in some of the cleanest, most oxygen-rich marine environments in the world, and protein skimming is one of the ways we can help recreate those conditions in a home aquarium.
Many healthy reef tanks use a protein skimmer - Photo by Shutterstock
Related article: Are hawkfish suitable for your reef tank?
How do protein skimmers work?
The basic principle is surprisingly simple.
A pump draws aquarium water into the skimmer and mixes it with air, creating a large quantity of tiny bubbles. These bubbles are passed through the body of the skimmer, giving them plenty of time to interact with the aquarium water.
Dissolved organic waste attaches to the bubbles, and the bubbles gradually rise towards the top of the skimmer.
As they reach the neck of the skimmer, the bubbles form a stable foam. This foam eventually spills over into the collection cup, where the concentrated waste accumulates.
The dirty liquid in the cup is called skimmate.
Empty the cup, and you've physically removed that waste from your aquarium. Easy!
Photo of Skimmate by Jamie Hendle
What is skimmate?
Skimmate is the brown, dirty liquid that collects in the protein skimmer's collection cup. It is concentrated organic waste that has been removed from your aquarium water. Reefkeepers often describe skimmate as either wet or dry.
Wet skimmate is lighter in colour and more watery, sometimes resembling lager.
Dry skimmate is darker and thicker, with a consistency more like gravy.
Most protein skimmers allow you to adjust the water level and air intake, giving you some control over how wet or dry the skimmate becomes. But there's no single perfect consistency for every aquarium. The important thing is that your skimmer is consistently removing waste and producing a stable foam.
As a general guide, aim to have the collection cup fill with dark, smelly skimmate at least once a week. The exact amount will vary considerably depending on the aquarium, its livestock, feeding regime and skimmer.
What makes a good skimmer?
When choosing a protein skimmer, two things are particularly important: bubble size and contact time.
A good skimmer produces lots of very fine bubbles and keeps those bubbles in contact with the aquarium water for as long as possible.
The smaller the bubbles, the greater the total surface area available for dissolved waste to attach to. Keeping those bubbles in the skimmer for longer also gives them more opportunity to collect organic compounds.
Protein skimmers use various designs to achieve this.
Many models use pinwheel impellers and venturis to draw air into the skimmer and break it into a fine mass of bubbles. Some also use diffuser plates and other internal components to further manipulate the bubbles and increase their contact time with the water.
Many modern skimmers have cone-shaped bodies, which gradually concentrate the foam as it rises towards the collection cup.
The exact design varies between manufacturers, but the principle remains the same: lots of small bubbles, plenty of contact time, and a stable column of foam.
Choosing the right size protein skimmer
Protein skimmers come in different sizes to suit different aquarium volumes, and manufacturers normally provide a recommended tank size for each model.
It's generally better to choose a skimmer with some capacity to spare rather than one that is only just large enough for your aquarium.
For example, if your reef tank holds 250 litres (55 gal), I wouldn't pick a skimmer rated for tanks of 250 litres (55 gal). Instead, go for the next size up, say one that is suitable for 500 litres (110 gal).
If you plan to keep a lot of fish, or have a heavily stocked reef aquarium, choosing an even larger skimmer can make sense.
The amount of waste produced by an aquarium depends not just on its volume, but also on how many fish it contains, how heavily you feed, and the overall bioload.
Larger skimmers are better of in a sump - Photo by Neil Hepworth
Related article: A guide to filtering your aquarium using a sump.
Keep your protein skimmer clean
Like any other piece of aquarium equipment, a protein skimmer needs regular maintenance. The collection cup should be emptied regularly, and the skimmer neck should be cleaned to prevent a build-up of waste and deposits.
A dirty skimmer neck can interfere with foam production and reduce the skimmer's performance. Keeping the neck clean allows the foam to rise smoothly into the collection cup. It's also worth checking the pump, air intake and other components periodically to make sure they aren't becoming blocked with salt deposits or debris.
Some skimmers are small and built to sit inside the aquarium, like this one by Sicce
Do I actually need a skimmer?
Not every reef aquarium has to use a protein skimmer, but they are an extremely useful tool and form the heart of the filtration systems on many successful reef tanks.
They're particularly useful on aquariums with lots of fish, heavy feeding or a high biological load, where there is plenty of dissolved organic waste to remove. They can also be valuable on smaller systems, where maintaining good water quality can be more challenging.
A protein skimmer isn't a replacement for good mechanical, biological, and chemical filtration. Instead, it works alongside them, removing dissolved organic waste and helping to keep the water clean and well oxygenated.
Once installed, give the skimmer time to settle in and learn how it behaves. Adjust it so that it produces a steady column of foam and regularly removes dark, smelly skimmate. Keep the collection cup and skimmer neck clean, and it will reward you with consistent performance.
A good protein skimmer won't do all the work for you, but it can make keeping a clean, healthy reef aquarium considerably easier.