Reef Refugium Setup Guide: Chaeto, Light, Flow & Harvesting

A refugium works best when you define the job before you buy a light, choose a chamber or add macroalgae.

A reef refugium can be used for:

• nutrient export through macroalgae growth and harvest;
• protected pod habitat and other microfauna;
• nighttime photosynthesis that may provide some pH support;
• additional biological diversity in a sump or separate vessel.

It is not a requirement for a successful reef tank. If you are still deciding whether you need one, start with protein skimmer vs refugium. If the decision is specifically between two algae-based export methods, use algae scrubber vs refugium. If you are choosing between an open chamber and a contained Chaeto device, compare macroalgae reactor vs refugium.

This guide assumes Chaetomorpha (“Chaeto”) as the default macroalgae because it is widely used, easy to harvest and does not need to attach to rock.

Reef refugium setup at a glance

Setup decision	What matters most	What not to assume
Purpose	defined job: export, habitat, pH support	every reef needs a refugium
Format	service access, safe plumbing, usable light placement	in-sump is always best
Startup	measurable nutrients and stable basic reef conditions	new tank = start Chaeto immediately
Size	enough usable growing/service space for your goal	fixed percentage of display volume
Light	plant-oriented output, safe mounting, timer	one universal PAR number
Flow	water exchange through/around algae, debris control	one universal turnover multiplier
Motion	healthy exposure and circulation	Chaeto must tumble
Photoperiod	repeatable schedule; reverse timing if useful	24/7 light is universally superior
Harvest	remove healthy excess biomass and preserve regrowth	more biomass is always better
Monitoring	growth + nitrate/phosphate trends + visible condition	color alone diagnoses the problem

Quick setup plan: match the refugium to the job

Goal	Start with	Measure	Success signal	Recheck if
Nutrient export	Measurable nitrate/phosphate, serviceable Chaeto space, safe light and a repeatable timer	Nitrate/phosphate trend, Chaeto regrowth and harvest interval	repeated Chaeto regrowth plus harvested biomass while the nutrient trend moves in the intended direction	Growth stalls, detritus accumulates, or nutrients become too limited to support the macroalgae
Pod habitat	Protected macroalgae/structure with safe water exchange and guards that keep loose algae out of pumps	Visible microfauna persistence and whether the refuge remains usable after normal maintenance	persistent microfauna in the protected area rather than a one-time seeding event	The chamber is repeatedly stripped, trapped detritus dominates, or pump/drain paths are unsafe
Nighttime pH support	Actively growing macroalgae plus a repeatable reverse or offset photoperiod	nightly low, daily high and daily pH swing before and after a schedule change	A repeatable moderation of the daily swing under otherwise comparable conditions	Indoor CO2, gas exchange or insufficient actively growing biomass remains the stronger limit

A refugium is not successful merely because the chamber contains green algae. Judge it against the job you defined: exported biomass and nutrient trend, persistent protected microfauna, or a measured pH response.

1. Define the job first

Nutrient export

If nitrate and/or phosphate is the reason for adding the refugium, measure them before startup and continue tracking them after startup.

Use a reef nitrate tester and reef phosphate tester appropriate for the range you are managing.

The useful sequence is:

1. Chaeto grows.
2. Nitrogen and phosphorus become part of new biomass.
3. You remove biomass.
4. The nutrients incorporated into that removed tissue leave the system.

That last step matters: harvesting completes nutrient export.

Pod habitat

A refugium can provide protected structure for copepods and other small organisms. Pod habitat does not require maximizing nutrient removal at all costs.

pH support

Lighting a refugium opposite the display can keep photosynthesis occurring during part of the display’s dark period. That can consume CO2 and can moderate the nightly pH drop if the macroalgae is actively growing.

It is not a guaranteed pH fix. High indoor CO2, weak gas exchange or a small macroalgae mass can limit the effect. If low pH is the primary problem, compare this approach with the reef CO2 scrubber guide.

2. Choose the refugium format

In-sump

An in-sump refugium uses a dedicated sump chamber or section.

Advantages include simple integration and easy gravity return in many systems. Trade-offs include light spill, competition for sump space and the risk that dense macroalgae traps detritus.

Do not sacrifice safe service access to pumps, skimmers or plumbing just to maximize refugium volume.

Hang-on

A hang-on refugium can add protected volume to a tank without a usable sump chamber.

Check mounting, pump path, return path, power-loss behavior, access and the extra leak risk created by any external water path.

External reactor

An external reactor confines macroalgae in a dedicated chamber with its own light and circulation.

Manufacturer instructions matter here. TUNZE publishes model-specific loading and flow guidance for its own Macro Algae Reactor. Those numbers belong to that reactor, not to every refugium.

Whichever format you choose, prioritize safe plumbing, harvest access, detritus removal, safe light mounting and protection against loose algae reaching pumps or drains.

3. Start only when the system has a job for it

Do not start a refugium just because the tank is new.

Chaeto needs nutrient supply to grow. Bulk Reef Supply advises waiting until a reef has measurable nitrate and phosphate before turning on a refugium.

Before startup, confirm stable salinity and temperature, measurable nutrients, sound plumbing and enough room to service the refugium after the rest of the equipment is installed.

4. Avoid percentage-based sizing rules

PetGearReport uses no universal refugium-size percentage.

Useful capacity depends on nutrient supply, light coverage, geometry, harvest interval, macroalgae growth and service room.

Instead of chasing a display-volume percentage, ask:

• Can I illuminate the usable algae area?
• Can water exchange through the biomass?
• Can I remove detritus?
• Can I harvest without dismantling the sump?
• Can loose algae be kept away from pumps and drains?

Why this guide does not use a universal refugium percentage or turnover rule

A fixed refugium-percentage rule of thumb is not a universal engineering requirement. The useful growing volume depends on the job, nutrient supply, light coverage, geometry, harvest interval and how much of the chamber can actually be serviced.

Likewise, an open sump chamber, hang-on box and sealed reactor do not share the same hydraulic path. A single turnover multiplier cannot describe all three. manufacturer model-specific flow guidance belongs to that reactor, not to every refugium format.

For an open refugium, tune for water exchange, debris control, algae containment and service access rather than chasing one display-volume percentage or one chamber-turnover number.

5. Choose a suitable refugium light

Purpose-built refugium fixtures such as Kessil’s horticulture/refugium products use a plant-oriented spectrum rather than treating the refugium like a coral display light.

That does not create one universal intensity number.

PetGearReport uses no universal PAR target for Chaeto. Useful intensity depends on fixture spectrum, distance, geometry, biomass thickness, nutrient availability and acclimation.

Mount the light so the manufacturer permits the environment, power connections stay dry, heat can dissipate and the lens or shield can be cleaned.

6. Put the light on a timer

Use a timer or controller so the photoperiod is repeatable.

TUNZE’s Macro Algae Reactor manual specifically tells users to set the illumination period with a timer and to avoid continuous lighting for that reactor.

Treat the exact duration as model-specific, not universal.

A practical tuning method is to choose a repeatable schedule, record growth and nitrate/phosphate trends, observe regrowth after harvest, and change one variable at a time.

7. Reverse photoperiod is a tool, not a guarantee

A reverse photoperiod means the refugium is illuminated during some or all of the display tank’s dark period.

TUNZE recommends alternating reactor and aquarium illumination for its MAR, and Bulk Reef Supply commonly uses the same approach in refugium setups.

The possible benefit is lower nighttime CO2 accumulation, but the magnitude depends on actual photosynthetic biomass and whole-system gas exchange.

8. Set flow for exchange, not a slogan

There is no universal flow-rate multiplier for refugiums.

An open sump chamber, hang-on box and sealed reactor do not require the same flow pattern.

The practical objective is to make sure water moves through and around the macroalgae while loose algae remains contained and the chamber does not become a detritus trap.

Signs flow may be too low include stagnant sections, heavy debris accumulation and dark interior tissue. Signs it may be too aggressive include repeated fragmentation and algae being forced into screens or plumbing.

If a return manifold supplies the refugium, use the aquarium return pump guide to separate real delivered flow from zero-head pump ratings.

9. Chaeto does not have to tumble

Tumbling is not universally required.

TUNZE’s own reactor documentation says that although circulation rotates the culture, the algae continue to grow without rotation.

For an open refugium, check light exposure, water exchange, debris removal and healthy regrowth rather than forcing a rotating ball.

10. Add a manageable amount of Chaeto

Do not fill the chamber completely on day one.

Leave room for water exchange, expansion, inspection and maintenance. Inspect new macroalgae for unwanted hitchhikers before it enters the system.

Avoid compressing the algae into a dense block.

11. Keep macroalgae away from pumps and drains

Use physical containment where necessary so strands cannot enter a return-pump intake, obstruct a drain, wrap around an impeller or block a sensor.

Any screen or guard becomes a maintenance item too.

12. Harvest for export and regrowth

As Chaeto grows, periodically remove healthy excess biomass.

Harvesting completes nutrient export because the nutrients incorporated into that tissue physically leave the aquarium.

Avoid never harvesting until the chamber becomes a shaded debris trap, but also avoid removing so much that very little healthy biomass remains.

If harvest frequency collapses, use the Chaeto refugium troubleshooting guide before automatically increasing light or adding supplements.

13. Clean the chamber

During maintenance:

• move the Chaeto enough to inspect the chamber;
• siphon settled detritus where practical;
• clean the light lens or shield;
• clean pump guards and screens;
• inspect hoses and fittings;
• remove nuisance algae that is smothering the intended macroalgae.

This is why service access matters more than maximizing every cubic inch.

14. Measure the system after startup

For nutrient export, track nitrate, phosphate, Chaeto growth and harvest interval.

For pH support, track the nightly low, daily high and the daily swing before and after changing the light schedule.

For pod habitat, look for persistent microfauna rather than assuming macroalgae growth alone proves success.

15. Do not solve every problem with supplements

Chaeto can use trace elements, including iron, but poor growth does not prove a trace-element deficiency.

Before dosing, verify nitrate/phosphate availability, light, flow, detritus, biomass density, competing algae and recent changes to other export methods.

The Chaeto troubleshooting guide covers that sequence in detail.

Setup checklist

• Define the refugium’s job.
• Confirm the system actually needs that job.
• Confirm measurable nitrate and phosphate if nutrient export is the goal.
• Choose in-sump, hang-on or external format.
• Check power-off behavior and leak paths.
• Ensure service access.
• Protect pumps and drains from loose algae.
• Mount the light safely.
• Put the light on a timer.
• Choose a consistent initial photoperiod.
• Establish enough flow for exchange and debris control.
• Add a manageable amount of healthy Chaeto.
• Record baseline nitrate, phosphate and pH if relevant.
• Plan how and when you will harvest.
• Change one variable at a time after startup.

Sources and methodology

PetGearReport has not physically tested refugium chamber sizes, measured PAR over Chaetomorpha, compared refugium fixtures, measured pod production, weighed harvest biomass or measured pH changes before and after a refugium installation.

Manufacturer-specific numbers remain attached to the exact product or reactor that published them rather than being generalized into universal reef rules.

• Bulk Reef Supply — Refugium Setup Tips for Nitrate and Phosphate Control — setup purpose, macroalgae choice, opposite-display lighting strategy and photoperiod as an adjustable variable.
• Bulk Reef Supply — Refugium Questions You Didn’t Know to Ask — measurable nitrate/phosphate before startup, detritus control and monitoring.
• Kessil — H160 Tuna Flora — example of a purpose-built adjustable plant-oriented spectrum fixture; product-design evidence, not a universal light prescription.
• TUNZE — Macro Algae Reactor 3181 and its Instructions for Use — macroalgae cultivation, harvest, circulation, timer-controlled lighting, alternating photoperiod and the manufacturer statement that algae can continue growing without rotation.

For deciding whether a refugium belongs in your reef at all, use the protein skimmer vs refugium guide. For stalled, pale, brown, slimy or fragmenting Chaeto, use the Chaeto refugium troubleshooting guide.

Frequently asked questions

How big should a reef refugium be?

There is no universal refugium-size percentage that fits every reef. Useful capacity depends on the job you expect the refugium to do, macroalgae growth rate, nutrient supply, light, flow, harvesting frequency and the space available for safe service.

How much flow should a Chaeto refugium have?

There is no universal flow-rate multiplier. The useful target is enough exchange that water moves through and around the macroalgae without turning the chamber into a detritus trap or blasting the algae into plumbing. Reactor manufacturers may publish model-specific flow settings; those should not be generalized to open sump chambers.

Does Chaeto have to tumble?

No. Tumbling is not universally required. TUNZE notes that its algae continue growing without rotation even though its reactor is designed to rotate the culture. In an open refugium, good water exchange, light exposure and debris control matter more than forcing a particular motion pattern.

Should a refugium light run opposite the display?

A reverse photoperiod is a common starting strategy because nighttime macroalgae photosynthesis can consume CO2 while the display is dark. It can moderate a daily pH swing, but it is not a guaranteed pH fix and the result depends on actual macroalgae growth and the aquarium's gas-exchange environment.

When should I start a refugium?

Start when the system has a defined need and enough nutrient supply to support macroalgae. Bulk Reef Supply specifically advises waiting for measurable nitrate and phosphate rather than starting Chaeto in a nutrient-starved new tank.