Best Refugium Setups for Aquarium Filtration
A refugium is a separate planted chamber — typically built into a sump or hung on the back of the tank — where fast-growing plants absorb excess nutrients from the water. Originally a saltwater concept using macroalgae, refugiums have gained popularity in freshwater aquariums as a natural nutrient export system. This aquarium refugium guide from Gensou Aquascaping at 5 Everton Park explains how to set one up.
What Is a Refugium?
A refugium is a lit, planted compartment in the filtration path. Water from the main tank flows through the refugium, where plants absorb nitrate, phosphate and other dissolved nutrients before the cleaned water returns to the display tank. It functions as a biological filter that uses plant growth to export nutrients — the opposite of traditional media-based filtration that traps waste mechanically.
Benefits of a Freshwater Refugium
Natural nitrate reduction: Fast-growing plants in the refugium consume nitrate continuously, keeping levels low without frequent water changes. Algae prevention: By removing the nutrients that fuel algae growth in the display tank, a refugium starves algae at the source. Fry and shrimp sanctuary: The refugium provides a safe space for fry and baby shrimp to grow without predation. Water clarity: Plants absorb dissolved organics that cause yellow tinting and odour. pH stability: Plant metabolism and gas exchange help buffer pH swings.
Refugium Designs
Sump refugium: The most effective design. Dedicate one chamber of your sump to a lit, planted section. Water flows from the overflow through mechanical filtration, into the refugium chamber, and then to the return pump. Size the refugium chamber as large as practical — the more plant mass, the more effective the nutrient export.
Hang-on-back (HOB) refugium: A commercial or DIY box that hangs on the back of the tank. Water is pumped up into the refugium and returns to the tank via overflow. Products like the CPR AquaFuge are available, or you can convert a large HOB filter by adding a light and plants. This option works for tanks without a sump.
Separate tank refugium: A small planted tank plumbed to the main tank. Water circulates between the two via a pump and return. This gives maximum flexibility in size and placement but requires more plumbing.
Best Plants for Freshwater Refugiums
Pothos (Epipremnum aureum): Not an aquatic plant, but its roots submerged in the refugium water are extraordinarily effective at absorbing nitrate. Many hobbyists consider pothos the single best refugium plant for freshwater systems. Simply place cuttings with the roots in water and leaves above.
Water Sprite (Ceratopteris thalictroides): A fast-growing floating plant that absorbs nutrients voraciously. Easy to harvest and dispose of excess growth.
Hornwort (Ceratophyllum demersum): Grows rapidly without substrate and absorbs large amounts of nitrogen and phosphorus.
Duckweed (Lemna minor): The fastest-growing option. Doubles in mass every few days under good light. Excellent at nutrient export but needs frequent removal to prevent it from blocking light.
Lighting the Refugium
A dedicated light over the refugium keeps plants growing. A small LED grow light or clip-on aquarium light is sufficient. Some hobbyists run the refugium light on an opposite schedule to the main tank — lit during the night — which stabilises pH by ensuring plant photosynthesis occurs around the clock. This 24-hour alternation provides more consistent CO2 and oxygen levels.
Maintenance
Harvest overgrown plants weekly or fortnightly. The nutrients locked in the harvested plant mass are permanently removed from the system — this is the actual nutrient export mechanism. Clean the refugium chamber periodically to prevent detritus buildup. Check flow rates monthly to ensure the refugium receives adequate water exchange. A refugium that receives too little flow becomes stagnant and ineffective.
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emilynakatani
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