What is macroalgae?
Macroalgae are large, multicellular marine algae, the seaweeds of the reef world. When hobbyists shop for saltwater aquarium plants, reef tank plants, or marine plants, macroalgae is almost always what they need. Strictly speaking they are not true plants: macroalgae have no roots, flowers, or vascular tissue, and they absorb nutrients directly through every surface of their tissue. That is exactly what makes them such efficient nutrient export. True marine plants such as seagrasses and red mangroves exist, but they grow far more slowly and remove far less nitrate and phosphate.
Aquarium macroalgae fall into three broad groups: green algae such as Chaetomorpha, Ulva, Codium, and Caulerpa; red algae such as Gracilaria, Halymenia (Dragon’s Breath), Hypnea, and coralline algae; and brown algae such as kelps and Sargassum, which are rarely kept in reef tanks. Nuisance film algae, diatoms, and dinoflagellates are microalgae, and cyanobacteria are not algae at all. Healthy macroalgae competes with all of them for the same light and nutrients.
Why keep macroalgae in a reef tank?
- Natural nutrient export. Macroalgae absorbs nitrate and phosphate as it grows. Harvesting a portion removes those nutrients permanently, which is why a refugium is one of the most dependable ways to lower nitrates in a reef tank.
- Nuisance algae control. A thriving macro culture outcompetes hair algae, film algae, and cyanobacteria by starving them of the nutrients they need.
- Copepod and amphipod habitat. Dense macros shelter breeding populations of copepods and amphipods, which drift into the display as live food for mandarins, wrasses, and filter-feeding corals.
- Steadier pH. Macroalgae consumes CO₂ while its light is on. Running the refugium on a reverse photoperiod offsets the overnight pH drop in the display.
- Fresh food for herbivores. Tangs, rabbitfish, and many angelfish graze Gracilaria, Dragon’s Breath, and Sea Lettuce, making a refugium a living feeder for your saltwater algae eating fish.
Macroalgae comparison: the best macro for every goal
Every macroalgae in this collection exports nutrients, but each species earns its place differently. Use this quick comparison to match a macro to your goal:
- Best all-around nutrient export: Chaetomorpha (Chaeto). Fast-growing, free-floating, and rarely goes sexual. The gold standard for tumbling in a refugium.
- Best nutrient export you can also feed: Red Ogo Gracilaria. Grows quickly and is one of the most readily eaten macros for tangs and rabbitfish.
- Easiest to harvest: Pom Pom Gracilaria. Holds together in dense clumps instead of shedding fragments into the sump.
- Best red color: Dragon’s Breath and the more intense Super Saiyan Dragon’s Breath. Brilliant red and orange for refugium viewing windows and display accents.
- Most unusual look: Atomic Broccoli. Branching red macro with an iridescent sparkle that shows best under white light.
- Best slow-growing ornamental: Blue Hypnea. Blue-violet, compact, and rarely needs trimming.
- Best for seahorse and pipefish tanks: Green Staghorn (Codium fragile). Sturdy finger-like branches make natural hitching posts for seahorses.
- Fastest leafy grower: Sea Lettuce (Ulva lactuca). Broad green blades that soak up nutrients quickly; harvest often to reduce the chance of a sexual event.
- Best for purple rock: Coralline algae. An encrusting, calcified red alga that seeds rockwork with pink and purple color.
- Best starter bundle: Refugium Essentials Macroalgae Pack. Chaeto, Dragon’s Breath, and Pom Pom Gracilaria together in one order.
By growth speed, Chaeto, Sea Lettuce, Red Ogo, and Pom Pom Gracilaria are the fast nutrient movers; Dragon’s Breath, Super Saiyan, Atomic Broccoli, and Green Staghorn grow at a moderate pace; and Blue Hypnea and coralline algae are slow, primarily ornamental choices.
Macroalgae to avoid or approach with caution
Caulerpa is one of the most searched macroalgae, and one we deliberately do not sell. It grows fast and removes nutrients well, but it roots into rock and sand, spreads through a display, and can go sexual overnight, clouding the water and releasing the nutrients it stored. Caulerpa taxifolia is listed as a U.S. federal noxious weed, and several Caulerpa species are restricted in some states, including California. Halimeda is a calcifying green macro that consumes calcium and alkalinity and grows slowly, so it suits displays better than nutrient-export refugiums. Whatever you keep, never release aquarium macroalgae into lakes, bays, or storm drains. Dispose of harvested macro in the trash or compost.
How to set up a refugium for macroalgae
Macroalgae grows best in a dedicated chamber, either a sump section or a hang-on-back refugium. Give it:
- Dedicated light. A refugium light with a full-spectrum or red-and-blue grow spectrum, run 12 to 16 hours a day, ideally opposite the display photoperiod.
- Gentle, constant flow. Enough movement to slowly roll Chaeto and Gracilaria so every side sees light. Dead spots trap detritus and shade the core of the mass, which is where melting usually starts.
- Measurable nutrients. Macroalgae needs some nitrate and phosphate to grow. A brand-new tank with near-zero readings will not support a large culture yet.
- A skimmer working alongside it. Macroalgae complements a protein skimmer rather than replacing it. The skimmer removes organics before they break down, and the macro mops up the nitrate and phosphate that remain.
Harvesting macroalgae and keeping nutrients balanced
Harvest whenever the culture fills its chamber or starts to shade itself, typically removing a third to half of the mass. That harvest is the nutrient export: whatever you pull out takes its stored nitrate and phosphate with it. Watch the other end of the scale too. A productive refugium can drive nitrate and phosphate to zero, which stalls the macroalgae and leaves corals pale and hungry. Track both with reliable aquarium water test kits and scale back the refugium light or harvest size if readings bottom out. Chaeto that stops growing while nutrients are still measurable is usually short on light, flow, or trace iron.
Keeping macroalgae in a display tank
Planted reef and macroalgae display tanks use ornamental reds and greens as living aquascape. Wedge new portions into rockwork or hold them with a rubber band until they attach or settle, keep them out of direct high flow, and trim regularly so they do not shade corals. Herbivorous fish will treat display macros as a salad bar, so in a tank with tangs or rabbitfish, keep a backup culture growing in the refugium. Chaeto and Sea Lettuce stay in the refugium; their growth is too fast and loose for most displays.
Growing your own fish food
Red Ogo Gracilaria, Pom Pom Gracilaria, Dragon’s Breath, and Sea Lettuce are all readily eaten by herbivores. A small cutting clipped to the rock makes a fresh, nutrient-rich treat that complements dried seaweed and prepared fish food. For help choosing grazers, read our guide to rabbitfish vs. tangs for algae control.
Portion sizes, shipping, and acclimation
Most macroalgae is sold by portion. Golf Ball is a starter portion roughly 2 to 3 inches across, Tennis Ball is about 3 to 4 inches, and Softball is about 5 inches; each product page lists the exact size for that species. Every order ships fresh from Reef Genetics cultures, and Sea Lettuce and Green Staghorn are grown in isolation from fish and invertebrates to reduce the risk of pests. On arrival, float the bag to match temperature, rinse the macro in tank water, look it over for hitchhikers, and place it a little farther from the light for the first few days.
Round out a healthy, natural system with a saltwater clean up crew and browse our live corals for sale.
