A refugium is a protected compartment connected to an aquarium. In a reef system it commonly houses macroalgae and small invertebrates away from display-tank predators. It can support nutrient management and food production, but it is optional and still needs maintenance.
What a refugium can do
Growing macroalgae takes up nutrients. Harvesting and removing part of that growth exports the nutrients from the system. Letting a crowded mass die and decompose returns material to the water instead.
Small animals can find shelter in the algae or rockwork and reproduce there. Some may enter the display, but the output depends on habitat, species and predation. A tiny refugium does not guarantee enough copepods for a mandarin dragonet.
The photograph shows brine shrimp, which are cultured separately for feeding. They should not be confused with a self-sustaining copepod population in a typical reef refugium.
A refugium adds some operating water volume, but it does not expand swimming space in the display. Account for substrate, equipment and the actual fill level when calculating volume; two containers’ advertised capacities do not simply add up to their working water volume.
Choose a layout you can service
A sump chamber, hang-on unit or separate connected aquarium can work. Choose accessible equipment that lets you inspect the algae, harvest it, remove debris and protect the return pump from loose material.
Aqueon’s sump instructions illustrate a protected refugium compartment for macroalgae and microfauna. Follow the plumbing and operating limits of your own equipment. Test drain-down and restart behavior so a stopped return pump does not overflow the system.
Keep loose algae away from drains and pumps, and block light spill that encourages growth in neighboring equipment. Provide water exchange throughout the algae without pinning it into a dense, stagnant mat.
Macroalgae and substrate
Chaetomorpha is a common starting choice because its wiry mass can be contained and harvested. Other marine macroalgae have different attachment, nutrient and lighting requirements. Identify what you buy rather than treating all algae as interchangeable.
A simple Chaetomorpha compartment can be kept without a deep substrate. Sand, rubble or mud should serve a deliberate purpose and remain manageable. No fixed two-inch mud layer guarantees complete denitrification, and a refugium does not need to be a deep sand bed to be useful.
Mangrove seedlings can be interesting plants, but their lighting, aerial growth and long-term space requirements differ from a compact macroalgae bed. Do not expect a few seedlings to replace fast-growing, harvested algae for nutrient export.
Light and growth
Provide a suitable algae-growing light and adjust its duration and intensity to actual growth. AlgaeBarn explains reverse photoperiod lighting: the refugium is lit while the display is dark. Photosynthetic CO₂ uptake can help moderate the overnight pH decline.
This does not guarantee constant pH or replace aeration and alkalinity management. A timer makes the schedule repeatable. Twenty-four-hour lighting is not automatically the best setting for every macroalga or system.
Track nitrate and phosphate rather than trying to remove every measurable trace. If levels become too low for the reef, reduce export appropriately. If algae stops growing, inspect lighting, flow, accumulated detritus and nutrient availability before adding supplements.
Maintenance and animals
AlgaeBarn’s Chaetomorpha care guidance emphasizes regular harvesting and checking the whole system when growth deteriorates. Thin crowded areas, remove deteriorating material and keep enough healthy growth to continue the culture.
A refugium intended to produce pods should not be filled with animals that eat them. Hermit crabs, starfish or temporarily housed fish can change that balance. None should be added on the assumption that an unlimited food supply exists.
A connected refugium is not quarantine. Shared water can carry pathogens and medication into the display. Use an independent hospital or quarantine tank when separation from the main system is required.
Refugium or protein skimmer?
A skimmer removes part of the organic load through foam collection and supports gas exchange. Macroalgae uses available nutrients to grow. The methods can complement each other; neither makes routine testing unnecessary.
Bulk Reef Supply’s worked sump setup includes both equipment and algae management. Choose the combination based on the reef’s measured needs, available space and maintenance capacity. A refugium succeeds through healthy growth, harvest and appropriate habitat, not simply by being present under the tank.
