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Preparation
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In the wild, fighting fish build bubble nests around pieces of vegetation at the water's surface. If you keep bettas, always have some floating greenery in your tank to encourage this natural behavior. Some fighting fish will start a nest anywhere, regardless of whether you lend a helping hand or not. Even if they're struggling to keep the nest together, do not intervene or attempt to place vegetation in the bubbles once they have begun building a nest.
Feeding
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During the nest-building period, feed your fish as you normally would and remove any uneaten food after 30 minutes if the fish don't seem interested in eating. Fish manage adequately without food for a few days, and they will not come to any harm. However, continue to offer food every day. Although the male will not leave the nest to search for food, if something drifts past him, he'll eat it.
Spawning
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Fighting fish are very protective of a nest and once it's complete, the male will lure the female under the bubbles to spawn and release her eggs. The male will gather the eggs in his mouth and blow them into the nest. The female will have no further involvement with the eggs or babies. The male will defend the area against potential predators until the eggs hatch.
Hatching
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After 24 to 36 hours, the eggs will hatch and the baby fish, also known as fry, will emerge. They'll use the bubble nest as shelter and protection from larger fish, and the male betta will continue to guard the nest and fry until they're able to fend for themselves.
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My Fighting Fish Are Blowing Bubbles But Not Eating
If you have a pair of Siamese fighting fish in your aquarium that appear to be preoccupied with blowing bubbles in a corner of the aquarium, they are building a bubble nest in preparation for spawning. Fighting fish, also known as bettas, may stop feeding for a day or two and become overly aggressive toward their tank mates at this time. A single male fighter will also regularly build nests even if a female fighter is not in the tank.