Now that your refugium is set up, it’s time for the fun part, deciding what goes inside it. A well-stocked refugium isn’t just extra space under your tank; it’s a living support system that helps keep your aquarium balanced and healthy over the long term.

One of the biggest advantages of a refugium is that it provides a safe place for beneficial organisms to thrive without constant predation. These tiny inhabitants play a major role in maintaining overall tank health. Copepods and amphipods are small crustaceans that help break down detritus and algae while also serving as a natural food source for many fish, especially smaller species. Over time, a healthy population of pods can even make its way into the main tank, contributing to more natural feeding behavior.

Alongside pods, a refugium often becomes home to a wide range of microfauna. Worms, small snails, and even bristleworms all help process waste and uneaten food, quietly supporting nutrient stability behind the scenes. In certain setups, mangroves can also be included. While they grow slowly, mangroves offer long-term nutrient export and oxygenation, and their root systems provide excellent shelter for microfauna as they develop.

Macroalgae are the real workhorses of most refugiums. They play a key role in nutrient control while also creating habitat for beneficial organisms. One of the most commonly used macroalgae is Chaetomorpha, often called chaeto. It grows in loose, tangled mats and is highly effective at absorbing excess nitrates and phosphates. Because it doesn’t go sexual like some algae can, it’s generally easy to manage and remains a favorite among aquarists.

Sea lettuce, or Ulva, is another popular option. This green macroalgae grows in thin, leafy sheets and does an excellent job of pulling nutrients from the water. Many hobbyists also appreciate that it can double as a supplemental food source for herbivores such as tangs and snails. It does tend to grow quickly, though, so regular harvesting is important to keep it from taking over the refugium.

Red Ogo, also known as Gracilaria, adds both function and visual interest. Its deep red coloration brings variety to a refugium while still offering strong nutrient absorption, particularly for nitrates and phosphates. Like sea lettuce, it can also be used as a food source for herbivorous fish, making it a versatile addition.

Pom Pom macroalgae, a form of Cladophora, is less commonly used but still worth considering. It grows in dense, rounded clumps that resemble small pom-poms and provides plenty of hiding places for pods and other microfauna. While it may not grow as aggressively as chaeto, it still contributes to nutrient export and adds texture and diversity to the refugium environment.

In addition to livestock and algae, the physical structure of the refugium matters. A thin layer of sand or gravel can encourage the growth of beneficial bacteria and give small organisms a place to hide and reproduce. Adding small pieces of live rock increases surface area even further, allowing bacteria to establish and helping break down ammonia and nitrites before they become a problem. Many aquarists also incorporate bio media into their refugium or sump to add even more surface area for beneficial bacteria and microfauna. Whether placed in a media basket or tucked into a low-flow area, bio media can quietly support biological filtration and provide additional habitat for pods and other tiny life.

At its core, a successful refugium is about balance. The right mix of beneficial creatures, macroalgae, and structure works together to keep nutrients in check, support biodiversity, and strengthen the overall health of your system. With a little patience and thoughtful setup, your refugium will become more than just a support zone, it can be one of the most effective and rewarding parts of your aquarium.