Refugiums: What Are They For?

Many times I see on the forums and Google Searches of beginners to saltwater aquariums asking ‘What is a Refugium’ and ‘What Does It Do?’

Refugiums are small tanks added to a reef aquarium that are used to grow micro-fauna and macro-algae that feed the livestock and help consume excess nutrients that nuisance algae use to grow. They can be easily installed in a sump or hung on the side of the aquarium.

These ingenious add-ons to your aquarium provide an area to grow algae out of sight of the main display and help prevent your glass turning green or have nuisance algae overtaking your beautiful aquarium.

What Is An Aquarium Refugium?

A refugium is an area dedicated to growing algae and microorganisms in a controlled way. The goal of a refugium is to replicate the natural reed beds found in most reef colonies in the wild.

Most reefers with a sump will dedicate an area specifically to a Refugium or ‘Fuge’ as it is most commonly known, or, for those aquarium owners without a sump, you can purchase ready-made Refugiums that can hang onto the side/back of a tank.

Hang On Refugium
Hang On Refugium
In Sump Refugium
In Sump Refugium

The Fuge is usually filled with either a Deep Sand Bed, ‘Miracle Mud’ or Ceramic Bio-Media Plates with various forms of macroalgae placed on top. The refugium is lit with a light that is of the correct light spectrum to encourage plant growth, thus allowing the algae in the Fuge to grow and consume nutrients at a rapid rate.

The more algae you can grow out of sight in the Fuge, the less nuisance algae you will have growing in your main display tank. The other main objective is to act as the ‘Refuge’ to the Microfauna. Microfauna are microscopic organisms that can play a vital role within the aquarium system.

They help to clean the water, consume nutrients & detritus, and feed the inhabitants in your display tank when the water flow catches them and flushes them into the main display tank.

Some examples of Microfauna are:

  • Copepods
  • Amphipods
  • Rotifers
  • Phytoplankton

What Do You Put In A Refugium?

There are 3 main things that go into a Refugium:

  1. Base/Substrate/Rock
  2. Macroalgae
  3. Microfauna

Lets talk about each one so you understand it’s role.


Deep Sand Bed – This is a layer of sand approx 6-8″ deep in the bottom of the Refugium. It allows Anaerobic Bacteria to colonize and convert Nitrates into Nitrogen gas which is slowly released.

This is an old school way and is slowly becoming less popular due to the fact that the sand bed will trap detritus over time and if the gas doesn’t escape, it could potentially build up to become toxic.

Live Rock Rubble is usually added on top of the sand to help provide additional surface area for the bacteria to colonize.

>> Find Substrates Here at <<

Refugium/Mineral Mud – This is a thick mud that you place on the bottom of the refugium. It is a great product for when you want to plant sea grass or mangroves into your Fuge. The mud is full of trace elements that help to promote the growth of the plant inserted into it.

>> Find Refugium Mud Here at <<

Bio-Media Blocks, Balls & Plates – These are a relatively new product released in the last 5 years and are really growing in popularity. They are a ceramic media that is full of tiny holes and passageways throughout each piece.

They are extremely porous and provide an enormous surface area for Anaerobic Bacteria to colonize, just like in the Deep Sand Bed. These, however, do not trap detritus like a Deep Sand Bed.
These are my recommendations for a refugium substrate.

>> Find Bio-Media Here at <<

Bare BottomMany Reefers like to keep their Refugium clean and have no substrate. The use of just Live Rock and a ball of Cheato is enough for them. While doing weekly maintenance they just blast off any settled detritus on the Live Rock using a turkey baster and that is all.


Cheatomorpha ( Click to see more at ) – More commonly known as Cheato, is a great algae for your Refugium. It is a fast-growing, high nutrient-absorbing plant. It grows into a thick mass similar to Wire Wool which creates a great habitat for growing Pods ( Microfauna ). Be sure to shake out the pods before disposing of half the algae ball – More on that later 😉


Caulerpa ( Click to see more at ) – Another very popular algae to add to your fuge. It is fast growing but will stick to your glass and Live Rock. It can come in many forms; Ferns, Grasses, Grapes etc. This plant, however though can stop reproducing and release the nutrients back into the water. Be sure to keep harvesting it to encourage it to stay sexual.


Mangroves ( Click to see more at ) – These are rapidly growing in popularity due to the high nutrient consumption abilities. Used with sand or the mud, they grow fast and are one of the most recognized aquatic plants in the wild. Just insert, let them grow and keep salt creep off their leaves and they are happy. No disposing of them.

Mangroves in Sump

There are many, many types of marine algae you can add to your refugium with great results. Above are the most popular within our hobby.


Microfauna are microscopic creatures that grow and populate with your aquarium and Refugium. You will see these little critters scurrying around or swimming, especially at night. A healthy microfauna population is a great thing to encourage in your system!

Copepods ( Click to see more at ) – These guys are by far the most common critter in our worlds oceans. They range from 0.2mm up to 10mm and there are 1000’s of species.
Copepods are great for consuming plant matter, decaying fish waste and nuisance algae such as diatoms.

They also get eaten by your inhabitants and a healthy ‘Pod” population is an absolute must if you wish to keep Mandarin Dragonette’s. It is all they eat and they consume Pods at a rate of one every 5 seconds, if they can find them!


Amphipods ( Click to see more at ) – Amphipods in your aquarium are the prawn-like looking creatures that grow up to about 10mm. These guys also feed on algae and detritus and help to reduce Nitrates in doing so. These are good meaty meal for any passer-by, they breed fast and are a great addition to your Fuge Pod Community!


Rotifers ( Click to see more at ) – Rotifers are one of the tiniest critters you may find in your tank. At 0.5mm in length they will appear as tiny specs of dust, but they are working! Their diet of dead and decaying matter make them a super janitor in all those microscopic places in your aquarium system.


Phytoplankton (Click to see more at ) – This species of marine fauna is made up of simple celled plant life on a microscopic level. Because they are plant based they photosynthesize so they consume Carbon Dioxide and expel Oxygen when subjected to light. They are fed on by the organisms listed above as well as filter feeding corals within your aquarium.


How Does A Refugium Work?

Water from the display tank is fed through the refugium part of the sump or is pumped into the ‘Hang-On’ Refugium, then fed back to the display tank.

The algae that is most commonly used in a Refugium is a Macroalgae called Cheatomorphia or ‘Cheato’ for short. This algae grows quick and does not stick to the glass. The algae that sticks to your rock and glass is a form of Microalgae.

The idea is to place a ‘Ball’ of this Cheato in your Fuge and let the water flow through it to allow the Cheato to consume the same nutrients that your nuisance Microalgae is also trying to consume.

A plant growing light is suspended over the Refugium, or placed against the glass in an All-In-One system (DIY modification required like this at to encourage the Macroalgae to grow as fast as possible to consume as many nutrients before the nuisance algae have time to consume them.

As the algae grows, you harvest and remove half of the ball or mass of Cheato and dispose of it. What you have now done is allowed the Cheato to consume the nutrients used by the nuisance algae and then physically removed them from the system. The Cheato now has more room to grow again and consume more nutrients.

Repeat and Repeat…

The Microfauna side of the Refugium is allowing these tiny organisms the space to reproduce without being picked off by fish, shrimp, crabs, and corals. Think of this as an organism factory. If you can keep a steady stream of organism growth, you will have tiny critters constantly working on cleaning the water, aiding the beneficial bacteria in your biological filter and then finding their way into the display tank as free food.

Does Every Aquarium Need A Refugium?

No, you do not! There are many successful saltwater aquariums out there that do not use a Fuge. Many argue a Fuge is not required and they may be right. BUT, there are many, many experienced aquarists out there who disagree.

I for one am an advocate of installing a Refugium because of the benefits it does bring. Many people say you need a ‘HUGE’ refugium for it to have any effect, but to me, having one is better than not having one and I had the room to easily add one.

There are some other benefits that having a Fuge will add to your system:

  1. The Cheato will consume Carbon Dioxide from the water and release Oxygen as part of its natural photosynthesis. Oxygen is always welcomed in a reef aquarium.
  2. It provides a safe haven the microfauna to populate and multiply without being eaten by your fish. As the pods get dragged out of the Cheato by the water flow, they end up in the tank as free food, or an endless supply of food if you have a Mandarin Fish and a healthy Pod population.

Can You Buy Or Make A Refugium?

You can do both! There are some awesome manufactured Refugiums out there in many shapes and sizes depending on your requirements.

You can view a good selection of Easy-To-Install Refugiums HERE at

These are super easy to install, light & run and will be a great benefit to your system.

As for DIY, the designs are endless! If you have an All-In-One system like a Biocube, Redsea Max or similar you can scrape off the paint off the back glass and install a light as these owners have done here at

You can purchase a sump with an area dedicated for a Refugium and use a small pump to move water to it or tap off a Return Pump Manifold to feed it.


You can create your own Fuge in your sump by having a glass piece cut and silicone it in to create your Fuge area. This is how I created mine, then I fed it off my manifold.

A Fuge does not need to be complex. Just a separate area with a little flow to grow Cheato and push your critters out into the main display tank.
You could even use a Rubbermaid container and have it drain into your sump. Anything will work!

What Kind Of Light Do You Need For A Refugium?

As with everything in this hobby you can find a cheap solution or an expensive solution to accomplish a given task. I have seen $10 setups work just as good as $300 setups, so it is up to you on how you want to install your Fuge. Some people want nice and shiny, and some people prefer Duct Tape and Ty-raps!

Many of the aquarium lighting manufacturers have lights dedicated to growing algae and they come in many shapes and sizes. Some will sit on top, some will be suspended over top and some can be stuck to the glass. Depending on your design, you will be able to find a solution that fits your layout and budget.

For my installation, I have a shelf sitting above my sump that holds my Aquarium Controller and all the items I need to keep out of reach of my children. This provided me the perfect place to install a fixed hanging light. I bought my fixture from Ikea for $15.

The bulb I use is a plant growth bulb which you can find HERE at and it works awesome. I get great growth from my Cheato. I then have my light on from 8pm to 8am using just a simple Plug-In-Timer like this:-

Plug-In Timer

Refugium Reverse Lighting Period – What Is That?

Photosynthesis is the natural cycle of plant life. Just like plants, when your aquarium inhabitants are awoken by the sun (Lights) they begin to consume nutrients and expel waste. Corals, aquarium inhabitants and algae are just the same.

During the day when your aquarium lights are on your tank life is consuming and expelling at a large rate, just as mother nature intended. Once the lights go out and everyone goes to sleep, the consumption rate reduces. What many aquarists see then is a rise in the Ph of their aquarium water.

By placing the lighting schedule of your refugium to be lit during the night it will allow photosynthesis and the awoken period of the microfauna to be alive and kicking. This will help to prevent the Ph from dropping as much. You will still get a Ph drop because your Fuge is nowhere near the size of your display tank, but again every little helps!

How Do You Install A Refugium?

The Fuge is meant to be as simple as possible. If it is a Hang-On type purchased from the store, you just hang it on the back of your sump or display tank, fill it, prime the pump, put the light on the timer, fill it with the goodies mentioned earlier and leave it to do its thing.

If its part of your sump then it even easier, just put in the goodies, light it and you are done!
It really is that simple and they add great benefits to your system.

Do You Need To Maintain A Refugium?

The only thing you need to do is every 2 weeks/month depending on how fast your Cheato grows is to rip the algae mass in half and throw that half away, that’s it, Finnito!

This is why adding a Fuge is an easy addition to make with great benefits and minimal work to you!

Further Reading

Are you looking to install a sump so you can add a Refugium? You can read all about sumps in my article:
Everything You Need To Know About Aquarium Sumps

Are you looking to add a sump to help reduce your Phosphates or Nitrates? These articles are full of great tips to help you lower both:
12 Ways To Reduce Nitrates In A Saltwater Aquarium
How to Lower Phosphates In Your Saltwater Aquarium
They worked for me!


Hi, I'm Richard and I have been an avid aquarist for over 30 years with a passion for Saltwater Aquariums. I love to pass on my knowledge to help others get the same amount a pleasure out of this hobby as I do. View my About Me page to find out more about me & my mixed reef aquarium.

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