How to Set Up a Refugium in Your Sump: Step by Step
A refugium turns your sump from a simple filter chamber into a living nutrient-export system that grows beneficial organisms, stabilises water chemistry, and reduces nuisance algae in your display tank. This refugium sump setup guide from Gensou Aquascaping at 5 Everton Park, Singapore, walks you through designing, building, and stocking a refugium that genuinely improves your aquarium’s health. Whether you keep a planted freshwater system or a reef, the principles are the same — grow something useful in the sump and let biology do the heavy lifting.
What a Refugium Actually Does
At its core, a refugium is a separate, lit compartment within your sump where macroalgae or aquatic plants grow. These organisms consume nitrate and phosphate from the water, effectively exporting nutrients that would otherwise fuel pest algae in your display. As a bonus, the refugium harbours copepods, amphipods, and other microfauna that reproduce and drift into the main tank as live food.
Think of it as adding a miniature ecosystem to your filtration. Mechanical and biological media handle ammonia and debris; the refugium handles the dissolved nutrients that media cannot touch.
Designing the Refugium Chamber
Most sumps have three chambers: drain inlet, media/equipment section, and return pump. The refugium typically replaces or shares the middle section. Allocate as much volume as practical — a larger refugium grows more macroalgae and exports more nutrients. For a sump under a 300-litre display, aim for a refugium compartment of at least 30-50 litres.
Baffles should direct water flow through the refugium at a moderate pace. Too fast and macroalgae get tumbled and fragment; too slow and detritus settles and rots. A gentle, steady flow — roughly 3-5 times the refugium volume per hour — is ideal.
Choosing a Substrate
A shallow bed of 2-4 cm of coarse sand or crushed coral gives macroalgae roots something to anchor into and provides habitat for microfauna. Miracle mud or mineral-enriched refugium substrates are popular in reef setups, though their benefits over plain sand are debated. For freshwater refugiums, standard aquarium soil works well for growing emersed or submerged plants.
Do not use fine sand that compacts easily — anaerobic pockets can develop and produce hydrogen sulphide. Coarse grain sizes allow water to percolate through without compaction.
Lighting the Refugium
Macroalgae need light to grow and consume nutrients. A dedicated grow light running on a reverse photoperiod — on at night when the display lights are off — stabilises pH by maintaining photosynthesis around the clock. This prevents the overnight pH drop common in heavily stocked tanks.
A basic 10-20 watt LED grow light with a colour temperature around 6,500K is sufficient for most refugium sizes. Mount it directly above the refugium chamber, 10-15 cm from the water surface. In Singapore, affordable clip-on grow lights are readily available on Shopee for $15-30 SGD.
Stocking With Macroalgae
For marine systems, Chaetomorpha (chaeto) is the go-to species. It grows as a tangled ball, is easy to harvest, and does not go sexual and crash the way Caulerpa can. Trim and remove half the mass every 2-4 weeks — this is your nutrient export. The removed chaeto can be discarded, composted, or shared with fellow hobbyists.
Freshwater refugiums benefit from fast-growing plants like Ceratophyllum demersum (hornwort), Salvinia, or pothos cuttings with roots dangling into the water. Pothos in particular is an aggressive nutrient sponge that thrives in Singapore’s humidity with its leaves growing emersed above the sump.
Adding Microfauna
Copepods, amphipods, and micro-bristleworms colonise a refugium naturally over time, but seeding speeds things up. Purchase a starter culture from a local reef shop or online seller — a small bottle of tisbe or tigriopus copepods costs around $10-15 SGD in Singapore. Pour them into the refugium, not the display, so they can establish a breeding population in safety.
These tiny organisms supplement the diet of mandarin dragonets, wrasses, and other planktivorous fish in marine setups. In freshwater, ostracods and daphnia can be cultured similarly.
Flow, Plumbing, and Overflow Tips
Route the overflow drain from your display into the refugium chamber first, then let water flow through baffles to the return pump. This ensures the refugium receives nutrient-rich water directly. Keep the refugium water level stable — fluctuations stress macroalgae and disturb microfauna. A well-designed baffle system with consistent overflow height handles this automatically.
Use a mesh screen or coarse sponge at the refugium outlet to prevent macroalgae fragments from reaching the return pump. Chaeto strands in your display tank are harmless but unsightly.
Ongoing Maintenance
Harvest macroalgae regularly — this is the entire point. If you let it overgrow, it chokes itself, dies back, and dumps nutrients right back into the system. Clean the refugium light monthly; salt creep and dust reduce output over time. Siphon detritus from the substrate every few months, but do not deep-clean — the microfauna living there are valuable. Gensou Aquascaping considers a well-maintained refugium one of the most effective and natural filtration upgrades any hobbyist can add to a sump-based system.
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