Growing macroalgae in a refugium has long been one of the reef hobby’s favorite natural approaches to nutrient management. Among the many macroalgae available, Chaetomorpha—better known simply as chaeto—has earned its place as a classic.

It may not be the showpiece of the aquarium, but behind the scenes, a healthy mass of chaeto can be doing plenty of work. As it grows, chaeto takes up nutrients from the water, creates habitat for copepods and other small organisms, and adds another living component to the reef’s overall ecosystem. Its wiry structure also tolerates the relatively strong water movement used in many refugiums and algae reactors.

The trick is keeping it growing.

A healthy green mass that steadily increases in size is what reef keepers want to see. Chaeto that stops growing, turns pale or yellow, becomes brittle, or begins falling apart is telling you that something has changed and figuring out what it needs is where a little refugium detective work begins.

Why Grow Chaeto in the First Place?

One of chaeto’s biggest jobs is nutrient uptake.

As the algae grows, it incorporates nutrients such as nitrogen and phosphorus into new biomass. But there is an important distinction here: growing chaeto captures nutrients; harvesting it is what actually removes those captured nutrients from the aquarium.

When you pull out a portion of healthy chaeto, the nutrients contained in that growth leave the system with it. That completes the nutrient-export cycle.

Chaeto also creates an excellent three-dimensional habitat. Its tangled strands provide protected surface area where copepods, amphipods and other small refugium inhabitants can live and reproduce with less pressure from hungry fish in the display.

For reef keepers trying to cultivate a healthy population of microfauna, that tangled green mass can become a miniature ecosystem all its own.

Light: Give Chaeto Something to Work With

Chaeto is photosynthetic, so good lighting isn't optional.

Green macroalgae use wavelengths across the photosynthetically useful spectrum, including red and blue wavelengths. Purpose-built refugium lights are designed with macroalgae growth in mind and can provide a more appropriate spectrum and intensity than simply relying on stray light from the display.

But more light isn't automatically better.

If chaeto isn't growing, increasing intensity or extending the photoperiod indefinitely won't solve the problem if something else, such as an essential nutrient, is limiting growth. Successful chaeto growth is a balancing act between light, available nutrients, water movement and the amount of algae you're trying to support.

Many reef keepers also run their refugium light on a reverse photoperiod, illuminating the refugium when the display lights are off. This keeps photosynthesis occurring somewhere in the system during part or all of the display's dark period and can help moderate the normal nighttime decline in pH.

Chaeto Needs Nutrients Too

It sounds almost backwards: we grow chaeto to help control nutrients, yet chaeto needs nutrients in order to grow.

That's exactly the point.

A healthy refugium isn't trying to create a completely nutrient-free aquarium. Chaeto needs access to nitrogen, phosphorus and other elements to build new tissue. If one or more essential nutrients become too limited, growth can slow or stop.

This can become especially noticeable in very low-nutrient systems. It is also possible to have nuisance algae growing in the display while chaeto struggles in the refugium. Different algae are competing for available resources, and nuisance algae already established across brightly lit rock and other surfaces may be very good at taking advantage of them.

Rather than automatically feeding more or adding fertilizer whenever chaeto slows down, test first. Look at nitrate and phosphate, consider the lighting and flow, and look at the system as a whole.

Macroalgae also require micronutrients, including iron, in small amounts. In some systems these can become limiting, but supplementation should be deliberate rather than a blind attempt to make the chaeto greener.

Flow: Around It, Through It and Sometimes Under It

Water movement helps continually bring nutrients and dissolved gases into contact with the algae and discourages debris from accumulating within a dense mass.

Chaeto's tough, wiry structure makes it capable of handling considerably more flow than many delicate macroalgae. The right amount, however, depends on the refugium itself.

And here's an important point:

Chaeto does not have to tumble to survive.

A gently tumbling ball can be beneficial because different surfaces are repeatedly exposed to light and water movement. But plenty of successful refugiums grow chaeto as a loose mass or mat without achieving the picture-perfect spinning green ball seen in videos.

If your chaeto is packed tightly together, collecting detritus or developing poorly lit areas inside the mass, improving circulation and thinning the algae may be more important than trying to make it perform laps around the refugium.

Don't Forget About Stability

Chaeto may be hardy, but it still lives in the same water as everything else in the system.

Stable salinity, temperature and general water chemistry provide a better environment for consistent growth than repeated swings. An automatic top-off can be especially useful for maintaining salinity as freshwater evaporates.

If possible, avoid directing a concentrated stream of freshwater top-off directly onto the chaeto. Allowing freshwater to mix with system water first reduces the chance of exposing a portion of the algae to a sudden localized salinity change.

Harvesting Is Part of the Job

Finding a refugium overflowing with chaeto might feel like winning but don't let success become its own problem.

As the mass gets larger and denser, inner portions can receive less light and reduced water movement. Detritus can accumulate, and portions of the algae may eventually decline.

Regular harvesting opens the mass back up and creates room for continued healthy growth. More importantly, harvesting is the step that physically removes the nutrients incorporated into that algae from your aquarium.

How often should you harvest?

There isn't one schedule that fits every reef. A fast-growing refugium may need frequent thinning, while another system may take much longer to produce enough growth to harvest. Let the growth rate of your own chaeto be your guide.

When Chaeto Isn't Happy

This is where reef keepers sometimes get frustrated. The chaeto looked fantastic…and then suddenly it didn't.

Rather than changing five things at once, work through the basics.

If chaeto has become pale or yellow, check whether lighting has changed and whether nutrients may have become limiting. If growth has stalled, look at nitrate and phosphate, lighting intensity and photoperiod, water movement, and whether the mass has become overcrowded. If the interior is dark or deteriorating while the outside remains healthy, self-shading and restricted flow are good places to start.

Brittle chaeto or strands that are breaking apart deserve attention as well. Check for recent changes in salinity, temperature, lighting, nutrients or other system conditions rather than assuming there is one universal cause.

And if the chaeto is packed with detritus, don't be afraid to gently clean and thin it during maintenance using aquarium water.

The key is to treat unhealthy chaeto as a symptom, not a diagnosis.

A Little Green Engine Behind the Reef

Chaeto isn't flashy. It doesn't fluoresce under blues, sway like a Euphyllia or become the centerpiece of the display.

But that's part of its charm.

Given adequate light, available nutrients, appropriate water movement and regular harvesting, Chaetomorpha can become a valuable part of a refugium taking up nutrients as it grows while providing a protected habitat for some of the smallest members of the reef community.

And when you pull back the refugium door and find a healthy green mass filled with tiny pods?

Sometimes the coolest ecosystems in a reef tank are the ones growing underneath it.