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A refugium (fuge) is a natural saltwater aquarium filter where macroalgae and copepods are grown to reduce waste and promote aquarium health.

A refugium can take your tank to the next level. Many aquarists today use more natural fish tank filtration like a refugium. A typical refugium setup includes a bed of fine sugar sand or miracle mud for beneficial bacteria, a macroalgae like chaeto, and some small live rock. A refugium light supports the macroalgae, which sucks up nutrients to prevent nuisance algae. The algae is pruned to remove excess nutrients. The sand and rock reduce ammonia, nitrite, and nitrate. Copepods eat waste and become fish food. A reverse daylight period helps stabilize pH.

The ideal refugium is at least 10% of the total water volume. An internal refugium sits inside the tank against the glass. A HOB refugium sits on the rim like a power filter. The most powerful choice is a dedicated aquarium sump refugium, or a large refugium chamber in an external sump.

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