Using Macroalgae in a Saltwater Aquarium Refugium

In this guide

True floating plants, the kind familiar from freshwater tanks, are not part of the saltwater hobby, since ordinary aquatic plants cannot survive in full strength seawater. The closest and most useful marine equivalent is macroalgae, grown either directly in the display or, far more commonly, in a separate refugium connected to the main system.

What a refugium is

A refugium is a separate section of the aquarium system, usually part of the sump, kept apart from fish and predators. It gives macroalgae, beneficial bacteria and tiny creatures like copepods and amphipods a protected space to grow, feeding benefits back into the main display without being eaten or disturbed.

Chaetomorpha, the standard choice

Chaetomorpha, commonly shortened to chaeto, is by far the most widely used refugium macroalgae. It grows as tangled, wiry green strands that do not attach to rock, which makes it easy to harvest and means it carries essentially no risk of spreading invasively into the display tank the way some other algae can.

Why keep macroalgae at all

• Nutrient export: macroalgae actively absorbs nitrate and phosphate from the water as it grows, giving you a natural way to keep nutrients in check alongside water changes and a protein skimmer.
• Pod production: the tangled structure of chaeto provides shelter for copepods and amphipods to breed undisturbed, which then drift into the display as a natural, ongoing food source for fish and corals.
• pH stability: running refugium lighting on a reverse schedule to the main display, so the macroalgae photosynthesises while display lights are off, helps smooth out the natural overnight dip in pH that most reef tanks experience.

Setting up a chaeto refugium

• Provide moderate to strong flow so the mass of chaeto gently tumbles, exposing all sides to light rather than sitting still and shading its own centre.
• Light with a dedicated, full spectrum refugium light for around 12 to 16 hours a day, run on a schedule opposite to the display's lighting.
• Start with a modest amount, roughly a fist sized clump, and let it establish before expecting significant nutrient export.
• Harvest a portion whenever the growth fills the available space, discarding the removed algae rather than returning it to the system, since that removal is what actually exports the nutrients it absorbed.

A caution on nutrient balance

Macroalgae needs some nitrate and phosphate available to grow, so in a very low nutrient reef tank, particularly one running heavy carbon dosing or an especially efficient skimmer, chaeto can stall or start to die back rather than thrive. If growth looks poor despite good lighting and flow, check whether your overall system nutrients have simply been pulled too low for the algae to sustain itself, rather than assuming a lighting or flow problem.

Ornamental macroalgae

Beyond chaeto, more decorative macroalgae such as red Gracilaria or blue Hypnea are sometimes grown directly in the display tank itself for colour and movement, either loose or anchored to rock. These need the same nutrient availability that chaeto does and are more prone to spreading if left unchecked, so they suit a keeper willing to prune regularly rather than a fully hands off setup.

Sourcing macroalgae in India

Chaeto and other refugium macroalgae are rarely stocked in ordinary local fish shops, since demand is limited to the smaller reef keeping community. Most Indian reefers source a starter culture either from a specialist online marine supplier or, just as commonly, as a small offshoot traded or gifted by another local reefer whose refugium has outgrown its space. Once you have even a small, healthy clump, it multiplies steadily enough that you are unlikely to need to buy it again.

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