Refugium Macroalgae Guide: Chaeto vs Caulerpa vs Gracilaria

Pick the wrong macroalgae and your refugium turns into a problem rather than a solution. Pick the right one and you've got nutrient export, pod habitat, and pH stability all in one chamber.

This guide compares the three most common refugium macroalgae and tells you when each one is the right call.

What macroalgae actually does

Macroalgae in a refugium serves three main functions:

• Nutrient export. Absorbs nitrate and phosphate as it grows. Harvest the excess, and you're physically removing nutrients from the tank.
• Pod habitat. Dense macroalgae provides shelter where copepods, amphipods, and microfauna breed safely.
• pH stability. Reverse-photoperiod lighting on macroalgae generates oxygen at night, balancing the pH dip from coral and fish respiration.

Different macroalgae do these jobs with different trade-offs.

Chaetomorpha (chaeto)

The default choice for a reason. Chaeto is a stiff, wiry green algae that tumbles freely in flow without attaching to surfaces.

Pros:

• Doesn't go sexual or release spores
• Doesn't sting corals or other livestock
• Excellent pod habitat (dense network of strands)
• Easy to harvest (just rip off a handful)
• Tolerates a wide range of conditions

Cons:

• Slower growth than caulerpa under low light
• Can mat together if flow is too low
• Doesn't add visual interest if your refugium is on display

Use chaeto if you want low-maintenance, reliable nutrient export and a pod factory. The right choice for 90% of refugiums.

Caulerpa

Caulerpa species (most commonly Caulerpa prolifera and C. racemosa) are leafy green macroalgae that root to substrate.

Pros:

• Fast growth, often faster than chaeto
• Good visual appeal
• Decent pod habitat in the leaf structure
• Some species are accepted by tangs as fresh greens

Cons:

• Goes sexual. Periodically releases gametes that turn the entire plant clear and dump nutrients back into the water. This can crash a tank.
• Spreads aggressively. Fragments establish elsewhere if they escape the refugium.
• Can produce caulerpenyne, a chemical compound that may stress some corals.

Caulerpa works if you stay on top of harvesting and keep light consistent (which discourages going sexual). Risky for set-and-forget refugiums.

Gracilaria

Gracilaria is a red macroalgae often sold as "tang heaven" food. It grows in branching, fleshy fronds.

Pros:

• Excellent fresh food for tangs and herbivorous fish
• Slow nutrient uptake, so it doesn't strip the tank
• Beautiful in display refugiums
• Doesn't go sexual

Cons:

• Slower growth than chaeto or caulerpa, so less nutrient export
• Less pod-friendly than chaeto's tangle of strands
• More demanding lighting
• Picky about flow

Gracilaria is best in tanks where you have herbivorous fish that benefit from continuous fresh greens. Less efficient as a pure nutrient export macroalgae.

Less common options

A few other macroalgae show up in refugiums occasionally:

• Halimeda. Calcareous green algae. Pretty, but doesn't grow fast and needs strong calcium dosing.
• Ulva (sea lettuce). Fast growing but messy. Sheds easily and clogs filters.
• Sargassum. Beautiful brown macroalgae but tropical and demanding.

For most reef-keepers, chaeto, caulerpa, or gracilaria covers the use cases.

Mixing species

You can run multiple species in the same refugium, but the trade-offs apply. Common combinations:

• Chaeto + gracilaria. Reliable nutrient export plus tang food.
• Chaeto + caulerpa. More visual interest, but caulerpa can outcompete chaeto.

If you mix, harvest the faster-growing species more aggressively.

Refugium light for each

• Chaeto. Full-spectrum LED, 6,500 to 10,000K, 16 to 24 hours. Reverse photoperiod recommended.
• Caulerpa. Slightly less light needed. 12 to 16 hours.
• Gracilaria. Stronger light helps. 12 to 16 hours of intense full-spectrum.

The bottom line

For most reef-keepers, chaeto is the answer. It does the nutrient export job reliably, supports a productive pod population, and won't crash the tank if you forget to harvest for a fortnight.

Pick caulerpa if you want fast growth and visual interest, gracilaria if you want fresh tang food, and consider mixing if your refugium is big enough.