Salty Q&A: Is a Sump the Same as a Refugium?

Aquarium sump - aquarium refugium


What exactly is the difference between a “sump” and a “refugium” (assuming there is a difference)? These are terms I didn’t hear in my 30-plus years as a freshwater aquarist, but I’ve heard them numerous times since starting my first saltwater tank.” – Submitted by Moira B


There is a distinction between a sump and refugium in marine aquarium hobby parlance, but sometimes there’s a degree of crossover between these two systems that can make it difficult to tell where one ends and the other begins. Let’s try to define each, and then briefly examine why they sometimes defy easy categorization.

Sump defined

A sump is a separate (but plumbed into the system) tank or reservoir, situated below the level of the aquarium, that is typically used to hold various life-support equipment—protein skimmer, heater, etc.—so it doesn’t detract from the aesthetics of the display. A sump also provides the added benefit of increasing the water volume of the overall system.

Refugium defined

A refugium is also a separate tank or reservoir that is plumbed into the aquarium system, but it serves a very different purpose. Essentially, a refugium offers a safe place—a refuge, if you will—in which to sequester organisms for one reason or another.

For example, hobbyists might want to create such an environment in order to culture microfauna (such as amphipods and copepods), bolster the system’s biofiltration capacity with additional live rock/sand, grow macroalgae for the purpose of nitrate reduction or feeding herbivorous fish, isolate injured or bullied specimens, or house interesting hitchhikers that may not be welcome in the display tank. I suppose one could say that a refugium usually enhances the biodiversity of the system in some way.

Where the lines blur

But as I mentioned above, sometimes the distinction between sump and refugium gets somewhat blurred. For instance, many hobbyists use multi-compartmented sumps, with some of the compartments holding equipment and others functioning as refugia. Also, even a rather “sterile,” strictly utilitarian sump can eventually begin to function as a refugium to a certain degree. If you look closely in my 29-gallon sump, especially at night, you’ll see a thriving population of pods and mysid shrimp (the latter were originally introduced several years ago as a live fish food)—probably because I’m lazy when it comes to siphoning out the detritus that settles there.


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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. Why can’t each be both, or to put it another way a refugium within a sump. I would suggest having a section of your sump to grow the likes of Cheato in it is a refugium where lots of things like pods can proliferate as well as various aliases and other creatures beneficial to our reef tanks can multiply.. IMO. A sump is just a vessel in line with the DT in which equipment can be put and to increase the total gallonage of the aquariums system. A refugium has little equipment like says a protein skimmer and it’s primary function is to grow algae’s and encourage pods plumbed in line with the DT. However, there is no reason why you can’t incorporate both within a sump as many do.

  2. Matt Bowers (Muttley000) says

    I have a system like what Atoll described where my sump contains a fuge section. One thing people,often do is banish a poor behaved specimen to the fuge, which likely make the fuge lose much of its effectiveness

    • Jeff Kurtz says

      Ah, good point, Matt! When you think about it, placing a misbehaving specimen in the fuge can be a lot like putting a fox in charge of the hen house (at least as far as pods et al are concerned).

  3. But is there any benefit one from the other? And how would you fit both?

  4. Christopher Schneider says

    Hello, I am working on planning a 75 gallon freshwater aquarium setup and I was wondering if I should get a refugium in my sump. I have no experience with refugiums and I am interested in learning if one would be beneficial in my tank. In the past I have used canister filters for my smaller tanks but I would like to explore the realm of sumps.

    🙂 Thanks!

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