Using a Refugium & Macroalgae to Stabilize Water Quality in Coral Reef Tanks

Maintaining stable nitrate (NO₃⁻) and phosphate (PO₄³⁻) levels is a perpetual challenge in coral tank management. Accumulation of these nutrients causes algae blooms, coral bleaching, and growth inhibition.

A refugium is an auxiliary tank separate from the main display tank where macroalgae are cultivated to biologically remove (export) nutrients through photosynthesis. While protein skimmers cannot remove all forms of nutrients, macroalgae absorb these compounds through photosynthesis to stabilize water quality.

How Refugiums Work

Macroalgae absorb nitrates, phosphates, and trace elements from the water through photosynthesis and grow. By regularly harvesting the algae, you physically export the accumulated nutrients outside the tank system.

The dense macroalgae in a refugium naturally cultivate copepods and other microcrustaceans (copepods, amphipods, etc.), which serve as natural food for the main tank inhabitants. Fish like clownfish and yellow tangs readily consume them.

Setting Up a Refugium

Location

The most common approach is to use one section of the sump (undercage filter) as a refugium. If space permits, you can also establish an independent tank (around 20–40 liters).

The key requirement is that water circulates between the main tank and the refugium. Water naturally circulates through the refugium section of the sump via pumps.

Lighting Setup

Macroalgae require sufficient light to grow.

Use dedicated refugium LED lighting (plant-growth spectrum, around 6,500K color temperature). Aim for moderate light intensity (PAR 50–150) sufficient for Chaetomorpha to thrive.

Reverse photoperiod lighting: Turn on the refugium lights during the night when the main tank lights are off.

Benefits: - Nighttime photosynthesis consumes CO₂ in the water, suppressing pH drop at night - Approaches 24-hour uniform CO₂ and pH control