Chaetomorpha: The Refugium Workhorse for Nitrate and Phosphate Export

Chaetomorpha, usually just called chaeto, is a tangled mass of thin green filaments that looks a bit like fine steel wool. It does not attach to rock or send out runners the way nuisance algae does. Instead it grows as a loose, tumbling ball, which is exactly why it works so well in a refugium. Water flows through the mass, the algae strips out dissolved nutrients as it grows, and you remove that growth on a schedule instead of watching it accumulate on your display rock.

Why Reefers Grow It

Nitrate and phosphate build up in every reef system from feeding, fish waste, and decaying detritus. Left unchecked they feed nuisance algae on glass and rock, and they can suppress coral colouration by fuelling excess symbiotic algae in the coral tissue. Chaeto competes directly for the same nitrogen and phosphorus compounds. Grow it deliberately in a refugium, harvest it regularly, and you are physically removing nutrients from the system every time you pull algae out, rather than relying solely on water changes or chemical media.

It also gives pods, copepods, and amphipods somewhere to live and breed, which is a nice secondary benefit if you run a mandarin dragonet, pipefish, or other pod-dependent fish.

Lighting and Flow in the Refugium

Chaeto is not fussy compared to most corals, but it does need real light to grow fast enough to matter. A small clip-on light or spare LED puck rated for freshwater plants will underperform. Reefers commonly repurpose a spare reef LED, or run a dedicated fixture from ranges like Aqua Illumination or Kessil, set to a blue-heavy or full-spectrum output somewhere in the range of 80 to 150 PAR at the surface of the water in the fridge or sump section housing the algae. Photoperiod matters more than intensity here. Many reefers run the refugium light on a reverse cycle to the main tank, lit while the display is dark, which helps stabilise pH swings overnight because the algae is photosynthesising and consuming CO2 while coral respiration is adding it.

Flow should be gentle but constant, enough to tumble the ball slowly so every part gets exposed to light and fresh water rather than shading itself in a dense clump. A small circulation pump aimed across the refugium chamber, or the return pump's own turnover through that section, is usually enough. Avoid flow so strong it mats the strands into a tight ball that shades its own core, since dead, decaying chaeto in the centre of a mass will actually leach nutrients back into the water instead of exporting them.

Harvesting Without Shocking the Tank

The trick with chaeto is removing enough to keep exporting nutrients without removing so much, so fast, that you crash the export capacity and the pod population living inside it. A practical rule is to never take more than about a third to half of the total mass in one sitting. Pull it out, gently rinse it in tank water over a filter sock or net to release trapped pods back into the sump rather than the bin, and leave the remainder to keep growing.

Doing this on a weekly or fortnightly rhythm, rather than letting it grow unchecked for months and then stripping the whole refugium at once, keeps nutrient export steady and avoids a sudden nitrate or phosphate spike as decaying trimmings release what they had absorbed. If you notice the mass turning brown, thinning, or shedding fragments on its own, that is often a sign of insufficient light or iron/trace element deficiency rather than a problem with the algae itself.

Sourcing Clean Stock

Because chaeto is grown in shared systems, always give it a good rinse and a visual check for hitchhiking pests such as bristleworms, aiptasia fragments, or unwanted snail eggs before adding it to an established refugium. A quick freshwater dip is unnecessary and can shock the strands, so a firm swirl in a bucket of tank water is usually sufficient.

Chaeto bags are a straightforward addition to almost any marine setup running a sump or hang-on refugium, and we regularly carry them alongside other nutrient management gear. Because livestock and plant availability shifts with each delivery, check out our stock list at our marine aquariums collection to see what is currently on hand before you plan your next refugium build.