Bubble nests are built primarily by labyrinth fish, a group of about 70 species across five families that includes bettas and gouramis. But they’re not the only ones. Certain armored catfish and even an African pike species also construct floating foam nests, making this behavior more widespread than most aquarium hobbyists realize.
Bettas and Gouramis: The Classic Bubble Nesters
The labyrinth fish suborder (Anabantoidei) contains the most familiar bubble nesters. These fish share a specialized breathing organ called the labyrinth, located in chambers above the gills, which lets them gulp air directly from the surface. This adaptation evolved in the warm, stagnant waters of Southeast Asia where dissolved oxygen runs low.
Among bettas, the Siamese fighting fish is the most widely recognized nest builder, but several other betta species share the behavior. On the gourami side, dwarf gouramis, pearl gouramis, honey gouramis, and three-spot gouramis all build surface nests. Males in all these species are the sole architects. Females play no role in construction.
Molecular studies of the labyrinth fish family tree show that bubble nesting evolved multiple times independently across different lineages rather than arising once in a common ancestor. Some closely related species within the same group took different paths entirely, with certain lineages evolving mouthbrooding instead of nest building.
Armored Catfish Build Nests Too
Outside the labyrinth fish world, the neotropical armored catfish Hoplosternum littorale builds one of the most elaborate bubble nests of any fish. Found in South American swamps and floodplains, this catfish constructs a floating dome of plant material supported by oxygen-rich foam. The nests average 30 cm (about 12 inches) in diameter and 6 cm (roughly 2.5 inches) tall.
Males begin building at night, and spawning typically happens around noon the following day. Each male defends a territory around his nest, using an enlarged pectoral spine to fight off intruders. Nearly half of all nests never receive eggs, which points to intense competition among males for mating opportunities. After hatching, which takes two to three days depending on temperature, the male guards the fry for only a day or two before abandoning them.
The African Pike: An Unexpected Nest Builder
The African pike characin (Hepsetus odoe) is a predatory freshwater fish found across sub-Saharan Africa, and it also builds foam nests. Its nest is irregularly dome-shaped, averaging 17 cm in diameter and 9 cm in height. The fertilized eggs actually rest above the water surface, sometimes as much as 3 cm above it, tucked inside the foam structure. Both adults guard the nest until the eggs hatch, then abandon it once the young attach to the bottom edge of the nest. The structure breaks apart about four days later.
How Fish Produce Sticky Bubbles
A regular air bubble popped at the water’s surface would last seconds. Bubble nest bubbles persist for days. The secret is mucus.
In Siamese fighting fish, researchers identified a specialized organ on the roof of the mouth called the pharyngeal organ, positioned right next to the opening of the labyrinth breathing chamber. When a male gulps air from the surface, that air passes over this organ and gets coated in a thick, glycoprotein-rich mucus before the fish exhales it underwater as a stable bubble. The mucus is highly viscous, which is what gives the bubbles their staying power.
This organ is sexually dimorphic. Males have a larger pharyngeal organ with significantly more mucus-producing cells than females, giving them a greater capacity to generate the sticky coating needed for construction. The wrinkled, textured surface of the organ expands the mucus-secreting area even further. It’s a body part that exists essentially for one purpose: building nests.
Why Bubble Nests Exist
Bubble nests solve a specific problem: keeping eggs alive in oxygen-poor water. Many bubble-nesting species live in shallow, warm, stagnant habitats where dissolved oxygen levels are dangerously low. Poor oxygenation has been linked to slower egg development, lower hatching rates, increased deformities, and higher mortality across fish species generally.
By placing eggs at or above the water’s surface inside a foam structure, males effectively lift the developing embryos into the most oxygen-rich zone available. In the case of Hoplosternum catfish, the nest’s primary function in the hypoxic water of tropical swamps appears to be exactly this: providing oxygen to eggs by raising them above the waterline while simultaneously protecting them from drying out. The African pike characin takes this even further, with eggs sitting several centimeters above the surface inside the dome.
The nest also serves as a physical barrier against egg predators and a visible signal to females that a male is healthy and ready to breed.
What Happens After Eggs Are Laid
In bettas, the spawning ritual is tightly choreographed. After the female releases eggs, the male collects each one in his mouth and places it into the nest. From that point, he becomes a full-time caretaker. He repairs popped bubbles, retrieves eggs or newly hatched fry that fall from the nest, and aggressively defends the area. He typically does this without eating or resting until the fry can swim on their own.
Betta fry begin dropping out of the nest roughly 36 hours after spawning, and the male constantly retrieves them and blows them back in. Once they become free-swimming and start moving horizontally through the water, the male’s parental instinct fades, and he may actually start eating the fry. In betta breeding, this is the point where the male gets removed from the tank. Feeding for the fry begins about two days after hatching, using microscopic foods.
Eggs in bubble nests are typically concentrated in the center of the structure, surrounded by an egg-free outer layer of foam about 1 cm deep. This cortex of empty bubbles acts as a protective buffer, enclosing the larvae even if the nest gets stranded or partially damaged.
Conditions That Trigger Nest Building
Male bettas and gouramis don’t need a female present to start building. A healthy male in good conditions will often blow nests on his own. But specific environmental factors make it more likely.
Water temperature is the strongest trigger. Bettas build most readily in water between 78 and 82°F, which mimics the warm, shallow waters of their native Southeast Asian habitat. Stable temperature matters more than hitting an exact number. Fluctuations can suppress the behavior. Calm water surfaces also help, since strong filtration currents break nests apart before they’re finished. Many betta keepers reduce surface agitation or provide floating plants to give the nest an anchor point.
Water chemistry plays a supporting role. Slightly acidic, soft water encourages nesting. Adding Indian almond leaves, which release tannins and gently lower pH, is a common technique among breeders. For the armored catfish Hoplosternum, the trigger is seasonal flooding. Males build nests in newly flooded swamps, particularly in open water along the edges, maintaining a minimum distance of about 10 meters between nests.