Why You Should Add a Refugium to Your Saltwater Aquarium

As you start to explore the saltwater aquarium hobby, you’ll probably encounter a recommendation to add a refugium to your tank. This simple equipment addition doesn’t seem like much (it doesn’t even take up a ton of real estate), and you may wonder what all the hype’s about. Or you may not even understand why hobbyists rave about having the little micro-ecosystem in their display tanks. Good thing you happened along here to find out what they are—and why you really want to add one to your aquarium.

Table of Contents: Adding Refugiums to Saltwater Aquariums

If you’ve already gained a little insight into refugiums, you may want to skip down to the link below on choosing an appropriate location. Or maybe you’ve never encountered the equipment before. That’s where the entire article will prove the most useful to you. Either way, I’ve laid things out to work for everyone.

Photo credit: Sean McGrath

What is a Refugium?

Let’s start with the basics in case you haven’t come across this vital piece of aquarium equipment just yet. A refugium is nothing more than an extra tank added to your aquarium, providing a refuge for primary producers (that would be bacteria, algae, and other plants) and primary consumers (copepods and other tiny crustaceans). Left out in the tank, these crucial food chain members would end up—well, eaten. So the tank protects them and allows them to grow and reproduce. When the populations “overflow,” they escape their home and drift out where they provide a meal for your fish and invertebrates. (And you can harvest the algae when it’s lush and crowding things)

If you want, you can think of the refugium as a nature preserve within the confines of your aquarium system. You’re setting aside a nurturing, PROTECTED sanctuary for the plants and animals inside. And since the tank’s attached to your display tank, the algae and copepods experience the water conditions in your aquarium. That means they absorb and provide the correct balance of nutrients when they eventually turn into fish food.

And if you find yourself with particularly delicate fish or coral species, that protected space works for them, as well. The plexiglass walls shield the animals from aggressive tank mates while they grow, but they still receive the water conditions present in the tank. It’s a handy bit of equipment that saves you from lining your entire house with aquariums (not that I want to stop you from that dream).

Why do refugiums work? Refugia are effective because they leverage natural biological and chemical processes to help maintain water quality and ecosystem balance. By providing a dedicated space for nutrient export and habitat partitioning, refugia allow beneficial organisms—such as macroalgae and bacteria—to process excess nutrients like nitrate and phosphate. Macroalgae absorb these nutrients through photosynthesis, while bacteria (especially in deep sand beds) facilitate denitrification, converting nitrate into nitrogen gas that escapes the system. This not only reduces unwanted nutrients but also helps stabilize the aquarium’s ecosystem.

Refugia also serve as safe zones for microfauna, enabling populations of copepods and amphipods to thrive without constant predation. These organisms contribute to the food web and help control pests. The refugium’s interaction with the main tank means that water, nutrients, and microfauna are continuously exchanged, integrating the refugium into the broader filtration and biological processes of your aquarium.

Common misconceptions and limitations: It’s important to note that refugia are not maintenance-free, nor are they a cure-all for water quality issues. Their effectiveness depends on proper sizing, flow, lighting, and regular upkeep. A refugium that is too small relative to the display tank, has insufficient flow, or is neglected can quickly become ineffective—or even a source of problems, such as nutrient spikes from decaying macroalgae. Overharvesting macroalgae or pods can also destabilize the system. Refugia work best as part of a holistic approach to aquarium filtration, complementing protein skimmers, mechanical filters, and regular husbandry.

Top Three Reasons to Add a Refugium

Now that you understand the reason behind that auxiliary tank, it’s time to look at WHY you might want to add one to your tank. Beyond saving you the trouble of setting up separate tanks to grow algae or nurture copepods, refugia offer three major benefits:

• Removal of nitrates from the water
• Addition of beneficial invertebrates
• Balancing of the water’s pH

Nitrate Removal

One of the biggest concerns aquarists focus on is their water quality. Keeping your water conditions in check tops your list. And one of the biggest helpers in that management is the bacteria and macroalgae you maintain in your tank. Every surface within the tank supports colonies of bacteria. And if you have a deep sand bed, anaerobic bacteria (those that thrive in the absence of oxygen) go to work on removing nitrates from the water.

How refugia remove nitrate: There are two primary mechanisms at play:

• Macroalgae uptake: Macroalgae (such as chaetomorpha or caulerpa) absorb nitrate and phosphate from the water as they grow, using these nutrients for photosynthesis. Periodically harvesting the macroalgae physically removes the absorbed nutrients from your system—a process known as “nutrient export.”
• Anaerobic bacterial denitrification: In refugia with deep sand beds, layers of oxygen-poor sand foster anaerobic bacteria that convert nitrate into harmless nitrogen gas, which then escapes into the atmosphere.

Trade-offs and operational factors: Macroalgae-focused refugia are generally easier to maintain and less risky than deep sand beds, which can become compacted and produce toxic hydrogen sulfide if not properly managed. However, macroalgae refugia require adequate lighting, flow, and regular harvesting to remain effective. The efficiency of nitrate removal depends on several factors:

• Refugium size: Larger refugia process more nutrients and support more macroalgae and microfauna.
• Flow rate: Sufficient water movement ensures nutrient-rich water reaches the refugium, but excessive flow can dislodge sand or delicate organisms.
• Lighting: Macroalgae require appropriate light intensity and duration for optimal growth and nutrient uptake.
• Macroalgae species: Some species are more efficient at nutrient uptake or easier to manage than others.

Potential failure modes: If macroalgae die off suddenly (due to neglect, lighting failure, or overharvesting), the nutrients stored in their tissues can be released back into the water, causing spikes in nitrate and phosphate. Deep sand beds, if not maintained, can compact and create anaerobic zones that produce hydrogen sulfide, which is toxic to aquarium life. Regular monitoring and maintenance are essential for long-term success.

Some hobbyists forgo the look of piles of sand on the bottom of the tank and create a deep bed in their ancillary tank instead. This provides the same benefits without compromising the style of the aquarium they wanted. Meanwhile, the bacteria go to work, scouring those unwanted nitrogen wastes from the water. All from a little extra tank.

And if you allow macroalgae to grow in the refugium, they’ll treat any excess nitrates in the water like plant fertilizer. So the more algae you grow, the more nitrate you will export from your aquarium system. Even better, as the macroalgae remove the nitrates, they strip the “nutrient” away from the microalgae you HATE seeing all over your live rock. Meanwhile, that confined refuge provides a “population control” by limiting the room your algae crop has to grow. The leaves shade themselves, halting growth. Then all you need to do is cut some away, feed it out, and the algae can grow again.

Invertebrate Boost

You’ll find tiny invertebrates (copepods and amphipods for the most part) in most mature saltwater aquariums. They like to hitchhike on live rock, hiding out in the crevices and natural caves. They’re a natural and highly nutritious food source for your fish. That isn’t a problem (it’s a good thing and important for many fish species). The fish in your tank will eat any pods they can catch. However, if you’re trying to prevent your pods from disappearing in a day? You need help.

A refugium provides the perfect oasis for these tasty morsels to grow and reproduce without getting munched. They have a delightful home among the algae (or bouncing around the sand), and they can watch the aquarium activity from the safety of the plexiglass. They also help out your algae by feeding on microscopic pests. That allows the algae to continue growing and removing nitrates. (See how that tiny ecosystem tank functions?)

Factors influencing pod productivity: The productivity of your refugium as a pod nursery depends on several factors:

• Refugium size: Larger refugia support bigger, more stable populations.
• Flow: Gentle flow allows pods to thrive and migrate to the display tank, but excessive flow can wash them out before they reproduce.
• Hiding spaces: Dense macroalgae and rubble provide shelter from predation and encourage breeding.
• Macroalgae density: More macroalgae means more habitat and food for pods.
• Predation: If fish or other predators can access the refugium, pod populations may not recover.

Limitations and expectations: While refugia can boost pod populations, small or poorly designed refugia may not produce enough pods to fully support high-demand species like mandarins or scooter blennies. In such cases, supplemental feeding or a larger refugium may be necessary. Overharvesting pods or macroalgae can also deplete populations, so it’s important to allow time for recovery between harvests.

Because the invertebrates don’t suffer massive predation all the time, their numbers increase. The little additional tank attaches directly to the display tank, so those extra pods make their way into your aquarium and boost the biological activity of your aquarium (and become part of the food chain). You create a normal ecosystem by allowing them to reproduce naturally (which leads to HIGH numbers).

pH Balance

Isn’t there a deodorant with the tagline, “pH balanced for women?” (Sorry, that was a tangent) A refugium won’t help you smell better, but it CAN help the pH balance of your tank. Many people set the light over that extra tank to operate when their display tank is dark. This allows the plants and algae (including symbiotic zooxanthellae in your corals and clams) to turn light and carbon dioxide into sugar, releasing oxygen. And—for those who remember high school chemistry class—the chemical reaction changes the pH in your tank.

How opposing lighting stabilizes pH: During the day, photosynthesis in your display tank consumes carbon dioxide (CO₂), which raises pH. At night, photosynthesis stops and respiration dominates, increasing CO₂ and lowering pH. By running the refugium light on an opposite schedule, photosynthesis continues in the refugium while the display tank is dark, consuming CO₂ and helping to buffer nighttime pH drops. This results in a more stable pH over a 24-hour period.

Limitations: This effect is only significant if the refugium is large enough and contains robust photosynthetic activity (lots of healthy macroalgae). Small in-tank refugia or those with minimal algae may have negligible impact on pH. Additionally, excessive light spill from the refugium can promote nuisance algae growth in undesired areas or disturb nocturnal tank inhabitants.

If you don’t use opposite lighting schedules, the pH of your tank will vary throughout a 24-hour period as photosynthesis occurs. Using this simple lighting trick can help balance those pH swings, making your tank more stable. And, of course, you’ll boost the growth of the algae within that little tank. That means you get to cycle back around to nitrate removal.

This only works if you have a refugium set up OUTSIDE of your display tank, though. And that brings us to the next important topic: where to place that little tank.

Picking a Location For Your Refugium

Okay, you’re convinced that adding a refugium to your saltwater aquarium makes sense. But now, you need to decide where to add your handy extra tank. You have a couple choices—and neither is better or worse than another. (The important thing is ADDING that little refuge in the first place) You’ll see three common places to connect a refugium to your aquarium:

• As part of the sump
• An in-tank option
• A HOB

How to choose? Here’s a quick comparison to help you decide which refugium location best fits your needs:

Operational consequences and best uses:

• Sump refugia are ideal for aquarists with sumps already in place and those wanting maximum nutrient export, flexibility, and the ability to run opposite lighting schedules. They are less visible but may be forgotten if out of sight, and can be more expensive to set up or expand.
• In-tank refugia are best for beginners, those with limited space, or anyone wanting to closely monitor their refugium inhabitants. They are simple and affordable but limited in size and impact, and may not allow for separate lighting schedules.
• HOB refugia offer a compromise—easy to add to most tanks, provide moderate water volume increase, and can often be fitted with their own light. However, they are limited in size by the tank’s rim strength, and may pose a risk of leaks or flow issues if not properly installed.

Most aquarists with a sump prefer to dedicate an area within their sump as the refugia. As part of the sump, your refugium connects directly to the aquarium’s existing plumbing, benefiting from the filtration system. You don’t need to pay for anything extra, and the filters keep your algae and pods healthy. It also works best if you want to use the opposing lighting trick. Unfortunately, it’s not a great choice if you happen to forget about the delicate fish fry you’re raising in the tank. Sumps reside UNDER your display tank—out of sight and sometimes out of mind. They can also get pricey, depending on how high-tech and complicated you decide to go.

If you prefer a simple option, in-tank refugium’s fit the bill. What you see is what you get: a small tank that attaches inside your display aquarium. You won’t need to break your bank, purchase extra equipment, or fret that you can’t see how your algae, fry, or pods are doing. Of course, you’re also out of luck if you decide you want a separate lighting schedule. And if you wanted some additional water volume for your tank? Yeah, not so much. But as a little safe haven, it works nicely.

Another popular and easy way to add a refugium to almost any tank is to use a Hang-on-the-back (HOB) style refugium. Similar to HOB filters, they attach to the back of your aquarium. A handy pump then moves water through the tank (and whatever pods feel it’s time to “leave the nest”). You gain additional water volume, and (depending on how you chose to set up your aquarium and stand) you might even be able to puzzle out a way to work on opposing lighting schedules. You WILL find yourself limited on size, considering water’s heavy, and you can’t expect to hang a monster tank from the back. But it’s a nice compromise between the other two options.

For More Information

Refugiums pitch in and make managing a saltwater aquarium a little easier. But if you feel like you need a little more convincing, let’s dive into some extra information. (Or, you know, add some icing to the cake, since you’re probably already placing an order for one)

This YouTube video will walk you through everything you need to know about refugia (and, yes, I promise it’s worth the watch):

https://www.youtube.com/watch?v=fzETnYKOImE

Or maybe you’d like to see a refugium in action? This YouTube video focuses on that crucial macroalgae harvest:

https://www.youtube.com/watch?v=yAmma223fi0

Interested in some fish that LOVE copepods? (That should boost your interest in adding one of these tanks!)

• Blue-spotted jawfish
• Fairy wrasse
• Lyretail anthias
• Mandarin goby
• Scooter blenny

Conclusion

Refugiums are a natural and effective way to improve water quality and ecosystem balance in your aquarium system. By providing a protected space for nutrient export and microfauna production, they help you maintain a healthier, more stable tank—and can even save you money over time by reducing the need for supplemental foods and chemical treatments. If you have the space and budget, adding a refugium is a practical upgrade for most saltwater aquariums.

Please note: There are affiliate links listed on the page. This means if you click on them (they take you to Amazon’s site) and buy something, a small amount of money is generated for me. I recommend, however, (for a complex purchase like this) that you buy from your preferred aquarium store so you can benefit from the helpful advice and information they can provide.

Written by Albert B. Ulrich III.