The Many Possible Purposes of the Saltwater Refugium

In-sump refugium with sand bed, live rock, & macroalgae

In-sump refugium with sand bed, live rock, & macroalgae

“Refugium” is a hobby term that resists easy definition. This is probably so because, unlike most aquarium accoutrements—heaters, powerheads, protein skimmers, lights, etc.—there’s no single, readily identifiable purpose for a refugium. Ask 10 different hobbyists what a refugium is for, and you’re apt to get 10 different answers.

Essentially, a refugium is some sort of tank, chamber, or reservoir that is isolated from the main display tank but shares the same system water. It could be another tank located under the display aquarium, a compartment within a sump system, a box that hangs on or inside the display tank, or one of many other possible configurations.

In broad terms, and as the name implies, a refugium can be said to serve as a place of refuge for an organism that is sequestered from the livestock in the main aquarium for one reason or another—but even that definition doesn’t exactly apply in every circumstance.

So let’s take a look at some of the more common uses for refugiums (refugia?) to get a better sense of what one is and why you might want to consider adding one to your aquarium system:

Isolating an injured/bullied specimen

A refugium is the perfect place to move a fish or coral specimen that has been injured/nibbled upon by a tankmate and needs a stress-free place to rest and recuperate in isolation. Because the refugium shares the same system water, you don’t have to worry about cycling it and matching parameters as you would with a completely separate hospital tank. However, for that same reason, you wouldn’t want to medicate a specimen housed in a connected refugium.

Culturing “pods”

Many fish appreciate snacking on amphipods and copepods, and some, such as dragonets and pipefishes, depend on these tiny live foods for their very existence. If you culture them in an isolated refugium, their population will have a chance to multiply without the pressure of constant predation, but some will still be swept or pumped into the display tank to become meals.

Nitrate reduction

This can be achieved in various ways with a refugium. One example is to use the refugium to hold a deep sand bed and/or several pieces of live rock. Anaerobic denitrifying bacteria that proliferate deep in the sand bed or inside porous live rocks can then convert nitrate to free nitrogen gas, which is released harmlessly into the atmosphere.

Another way is to grow macroalgae, such as Chaetomorpha, in a lighted refugium. The algae will take up nitrate and phosphate to fuel their growth and can then be harvested on a routine basis to export these nutrients from the system.

A home for interesting hitchhikers

Certain live rock hitchhikers, such as crabs and mantis shrimps, may not be welcome in a display aquarium, but sometimes they can be pretty cool to observe in their own right. If you don’t want one of these uninvited guests in your tank but find it kind of fascinating, keeping it in a refugium may be a perfectly viable alternative to dispatching it or giving it away.

How do you use a refugium?
So fellow salties, if your saltwater system includes a refugium of some sort, what purpose did you have in mind when you set it up and how has it performed for you? Let us know in the comment section below.

Photo credit: Saltwater Smarts


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About Jeff Kurtz

Jeff Kurtz is the Co-founder/Editor of Saltwater Smarts, former Senior Consulting Editor for Tropical Fish Hobbyist Magazine, and the aquarist formerly known as “The Salt Creep.” He has been an aquarium hobbyist for over 30 years and is an avid scuba diver.


  1. My refugium is kind of a refugium/algae trough that sits above the water so it is able to take advantage of the reef lighting and it also enjoys the water that comes out of my skimmer. Now it is filled with tiny tube worms and is loaded with amphipods, copepods and brittle stars. I find it more interesting than my main reef.

  2. Matt Bowers (Muttley000) says

    Another benefit of a separate refugium is the increased water volume helps insulate from temperature or chemistry swings. I have however heard people incorrectly justify a species in too small a tank because they are placing in an acceptable “system size.”

  3. I use a refugium (DSB/macro algae/rock ruble) for nutrient export and culturing pods. It also increases water volume to help keep parameters more stable. Another benefit to having macro algae is that I can harvest it and it give some fish a tasty treat that they are used to in the wild.

    • Jeff Kurtz says

      Thanks for your insights, Hans! That’s a great point that the macroalgae grown in a refugium can be a tasty treat for the fish too!

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