A refugium is the single best upgrade most reef-keepers can add to their tank. Done right, it's a copepod factory, a nutrient sink, and a biodiversity engine all at once. Done wrong, it's just an extra bucket of saltwater taking up space in your sump.
This guide walks through how to set up a refugium that actually produces pods at the rate your reef needs.
What a refugium does
A refugium is a sheltered space, usually in your sump, where microfauna can breed and macroalgae can grow without being eaten or hammered by flow. Pods born in the refugium drift out into the display through the return line, where fish and corals feed on them.
It also pulls nutrients out of the water column. Macroalgae like chaeto absorb nitrate and phosphate as they grow, which lowers your overall nutrient load and reduces nuisance algae in the display.
Sizing the refugium
The general rule: aim for a refugium volume of around 10 to 20 percent of your display tank. Bigger is always better when it comes to pod production. A 200 L display should be running at least a 20 L refugium, and 40 L would be ideal.
If your sump is small, a hang-on-back refugium or a separate small tank plumbed in via a low-flow pump are both viable options.
Lighting
Refugium light needs to support macroalgae growth without bleaching it. The specs that work:
- Full-spectrum LED in the 6,500 to 10,000K range
- 16 to 24 hours per day (many reef-keepers run reverse photoperiod, lighting the refugium when the display is dark to help stabilise pH overnight)
- Position close enough that the macroalgae actually grows. If chaeto isn't doubling in size every few weeks, your light isn't strong enough.
Substrate and rockwork
Skip a deep sand bed unless you specifically want to run one. For copepod production, what you want is:
- Live rock rubble. Fist-sized pieces give pods crevices to hide and breed in. The more surface area, the better.
- Macroalgae. Chaeto is the standard. It tumbles freely, doesn't sting, and shelters huge pod populations. Caulerpa works but goes sexual occasionally and crashes. Gracilaria is good if your tang likes it.
- Optional: thin sand layer. A 1 to 2 cm sand layer supports more types of microfauna without going anaerobic.
Flow
Refugium flow should be slow. Slow enough that chaeto tumbles gently, not slow enough to settle. A small powerhead pointed at the macroalgae works, or just rely on the slow circulation from the sump return.
Fast flow blows pods straight out of the refugium and into the display before they can breed. Keep it gentle.
Stocking
Once the refugium is set up and the macroalgae has been growing for a couple of weeks, seed it with copepods. A mix works best:
- Tisbe biminiensis for surface and substrate dwellers
- Apocyclops panamensis for pelagic, water-column pods
- Amphipods if you want larger prey for predatory fish
Dose in late evening with the lights off. Don't add a hungry mandarin or pipefish until the refugium has been producing for at least 4 to 6 weeks.
Maintenance
A productive refugium needs surprisingly little hands-on maintenance:
- Harvest macroalgae when it fills the chamber (every 2 to 6 weeks depending on growth rate)
- Top up pods if you've got heavy predation in the display
- Dose live phytoplankton occasionally to feed the pod population directly
- Check that flow hasn't pushed all the chaeto into one corner
The bottom line
If you have the space, build a refugium. It's the single most cost-effective way to support a healthy reef long-term. Pod production, nutrient export, biodiversity, pH stability — all from one chamber.
