Good Reasons to Use a Refugium in Your Reef System

Macroalgaes (such as Codium), commonly grown in refugiums, help clean water

Macroalgaes (such as Codium), commonly grown in refugiums, help clean water

A refugium is a safe haven for algae and invertebrates in a separate tank from the main display aquarium. These organisms provide some distinct advantages to the overall health of a reef aquarium. First off, the critters breeding in the refugium occasionally make their way back to the display and offer healthy snacks for many of the fish and corals. Second, the critters and algae in the refugium help stabilize water chemistry and process nutrients.

These benefits are not possible when there is too much predation on microfauna and algae—as is typically the case in a display tank. For example, the tangs and other herbivorous fish that are popular in reef aquariums make short work of macroalgae. Also, fish such as damsels, mandarins, and wrasses like to constantly pick off tiny copepods and smaller planktonic life. Over time, the beneficial algae and critters would just disappear.

Beneficial invertebrates

Beneficial inverts that proliferate in refugiums include a mix of various amphipods, copepods, mysis shrimp, mini brittle stars, and worms, which include bristleworms, feather dusters, and spaghetti worms. These inverts are detritivores. They consume uneaten food and waste from the main tank. In doing so, they break down waste and convert it to biomass, thus making it unavailable to undesirable things like cyanobacteria.

refugium1Also, as I mentioned, a small fraction of these inverts make their way back to the display tank where they become healthy snacks for fish. Many of the fish that are popular in reef aquariums forage for food constantly and are ill served by a once-per-day feeding regimen. A large refugium constantly supplying a food source greatly helps in this regard. Some of the healthiest animals I have ever seen came from massive systems where the tank itself was able to keep up with the feeding activity of fish. However, not everyone has the space for a giant system with only a couple fish in it.


Macroalgae, which are commonly grown in refugiums, help clean the water by binding up nitrates, phosphates, and heavy metals. Periodically harvesting the algae from the system effectively removes these compounds. There are even entire filtration systems designed around the water-cleansing properties of algae.

Most of the time, reef hobbyists focus on the most productive algae—those that are fast-growing and bind nutrients best—for filtering their systems. Some argue that the most productive are turf algae. Turf algae are relatively uncommon in the hobby and require a flow-heavy environment for best growth. A type of filter called an algal turf scrubber (similar in concept to this DIY), or ATS, was made to grow and harvest turf algae; however, not many hobbyists use them.

Second to turf algae, the most popular alga used for filtration purposes is Chaetomorpha. It is a stringy, green alga that grows in dense clusters.

Reverse-daylight photoperiod

A typical reef tank’s pH fluctuates throughout the day as a function of photosynthetic activity. At night, the pH tends to dip compared to the day. A reverse-daylight photoperiod—in which the refugium is lit while the main tank is dark and vice versa—helps stabilizes tank pH. By doing this, the refugium’s algae resume photosynthesis where the main tank stops and, thus, balance out the pH. Furthermore, stabilizing pH helps the system maintain other chemical parameters.


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About Than Thein

Than Thein is the owner of Tidal Gardens and Advanced Reef Aquarium. Than's love for all things underwater began early on when dogs and cats were strictly off limits, but a fish tank? Sure! What started with a 10-gallon goldfish tank eventually turned into a 5,000-gallon greenhouse coral propagation system. In addition to coral aquaculture, Than's other hobbies include scuba diving and underwater photography and videography.


  1. Another great article I just started to build a refugium system thanks guys you all just gave me some great ideas
    Keep up the good work
    Thank you

  2. Paul Baldassano says

    I love codium in my main display. There are very few real seaweeds one can get to add some color to the tank. I ciollect codium out on the east end of Long Island in the summer as it is very common there near shore

  3. Chris Lawson says

    What’s you opinion on collecting macro algae from the ocean? In our local waters around Sydney Australia there are beautiful macro algaes, some are even blue in colour. Local ocean surface temperatures range from about 18’C to 23’C (64 to 73 F). Rock-pool water would reach over 30’C in summer. There are also readily accessible various sponges and crustaceans. I’ve always been tempted to try some in my refugium or even the display tank. Would I be tempting fate / disaster?

  4. Paul Baldassano says

    I have been collecting them in New York for 40 years and our water is much colder than yours, so the local Macro doesn’t live more than a few months. But it is free and easy to collect so every summer I get a bunch. It looks fantastic because you never see it in anyone’s tank.

  5. Hi guys love your utube video I have been thinking of getting a hang on the back refugium for my 100l tank but my pet shop said because my tank is only 100l I do not need one what do you guys think? and do you have a list of macro algaes that I could keep in my main tank that would not take over? thank you

    • Chris Aldrich says

      Hi David – I’ve seen HOB refugiums on tanks smaller than yours, so if you’re interested in one and can make it work with your setup, I’d say go for it. Macro algaes can be kept in the display, but as you mentioned, controlling their spread could be an issue depending on the variety. Also, some macros are readily eaten by tank inhabitants which would make it harder for them to thrive.

      If you’re interested in the full benefits of a refugium, look into a HOB or plumb a seperate one in. You can absolutely do either with your system.

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