Five Reasons to Install a Refugium on Your Saltwater Aquarium

By PetMD Editorial on Feb. 11, 2019

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By Kenneth Wingerter

As our saltwater aquarium animal collection grows in size so should the fish aquarium. Of course, that is not always so simple. For example, you might have a strictly limited amount of floor space to work with. So, what to do?

One popular way to add precious water volume to an established system is by adding a sump or refugium.  


Differences Between Sumps and Refugia

From the onset, it's helpful to understand the differences between sumps and refugia. Yes, they both serve as reservoirs for recirculating aquarium systems. And they are, indeed, plumbed in the same general manner. A sump might also contain a refugium or vice versa, but that is about where the similarities end.

The most notable distinction between the two lies in their fundamental purposes. Sumps are used mainly to centralize and compartmentalize filtration and monitoring equipment. Refugia, on the other hand, are used primarily to promote the growth and reproduction of targeted plants (usually macroalgae) and animals (especially copepods).

In many cases, refugia truly provide a place of refuge for "macros" and "pods" that otherwise get overharvested in the main tank through intense predation/herbivory. 

What's in a Refugium?

The original and most popular style of refugium consists of a large reservoir (often a second fish tank) with a deep sand or gravel bed. A dense carpet of seaweed (e.g. Chaetomorpha) grows over the bottom. For the purpose of growing the seaweed, a powerful, full-spectrum lighting system (such as the Current USA Orbit marine aquarium LED light) is used.

Copepods thrive in this environment. Not only is the space within the algal mass perfectly sheltered, but the mass itself also provides an enormous amount of habitable surface area.

And, most importantly, the pods can grow and multiply in a space that is free of predators. Think of a refugium as a micro marine reserve for pods. Progeny from a highly productive population of copepods in the refugium spill out into the main tank (and the hungry mouths of fishes and corals). 

Freshwater aquarists have just begun to experiment with this arrangement using aquatic plants (e.g. Stuckenia) and freshwater amphipods (e.g. Hyalella).

Benefits of Installing a Refugium

Sure, there's a lot of stuff going on in the typical refugium. The natural processes that unfold within them certainly may seem interesting. But is the effort of installing a refugium really worth it? Why might an aquarist want to install one?

Here we weigh all the major benefits of using a refugium and present five big reasons you should consider one for your own aquarium system.

  1. Nitrate Control

The deep sand bed hosts a variety of highly beneficial anaerobic bacteria (denitrifying bacteria, purple non-sulfur bacteria) that metabolize nitrate. By lowering nitrate levels, they serve to limit the growth of nuisance algae. Particularly where the main tank has a thin bottom cover or bare bottom, a refugium with a deep bed of sand (such as Nature's Ocean Bio-Activ Live Aragonite saltwater aquarium sand) adds a tremendous amount of habitat complexity to the system.

  1. Even More Nitrate Control

Macroalgae take up nutrients, such as nitrate, as they grow. In so doing, they compete with "bad" microalgae that form unsightly films and turfs. The idea is to use the macroalgae as a vehicle for nutrient export so unwanted algae can’t grow.

As the good algal mass grows large enough to fill the confines of the refugium and starts to shade itself, growth rates decline. It is at this time that a portion of the mass (along with the "absorbed" nutrients) is harvested and discarded. Certain palatable types of seaweed (e.g. Ulva) can be fed out to herbivorous fishes and invertebrates in the main tank.

  1. Copepod Nursery

In addition to helping to maintain good water quality, macroalgae provide an excellent habitat for microcrustaceans, such as copepods. The macroalgae love the pods, which keep them clean and make them able to capture as much light as possible. As older plant material declines, deteriorates and starts to decay, copepods will surely be there to feed on the waste.

With perfect living conditions and no fish present to feed on them, pods in the refugium can be extremely productive. Just as in the wild, these tiny animals are important intermediaries in the aquarium food chain. It's pretty difficult to overstate their value; they essentially turn bad stuff (nuisance algae and organic waste) into a super-nutritious live fish and coral food.

  1. Sediment Pooling

While no one really wants organic muck anywhere in their aquarium system, we'd all agree that it's better tucked away in a refugium than plainly visible in our display.

If refugia are heavily planted, they act as sediment sinks. That is, particulate organic matter that passes in from the main tank tends to settle out in the refugia. This is due to the slowing of water currents as they pass through the dense patch of seaweed. The settled detritus accumulates on the bottom, where it is subsequently consumed by copepods and other deposit feeders.

  1. Water Volume Gain

Merely by adding a bit to the overall water holding capacity of your system, you're giving the aquarium livestock a little more room to breathe. Whether freshly set up or fully mature, any system benefits from the additional water volume.

But in the case of a refugium, you're not just adding volume; you are greatly diversifying the larger captive ecosystem, which allows for more biological diversity. While it's one thing to just add dead space (as in a sump), a refugium creates a hot spot of biological activity, where waste products are ultimately converted to copepod and macroalgae biomass.

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