What Happens When Fish Lay Bubbles on the Top of the Water?

If you've seen a mass of bubbles floating atop the water in your pet fish's bowl, that's not evidence that he needs a water change. They're actually a sign he's ready to start a family. Many types of fish -- including bettas, paradise fish, labyrinth fish and gouramis -- build bubble nests for their eggs and the young fish that hatch from them. It's the male fish who create the bubbles and construct the nest.

  1. Nest Construction

    • When a male fish feels his biological clock ticking, he'll build a nest of bubbles before he even starts courting a mate. He does this by swimming to the surface of the water, drawing in a mouthful of air and then ejecting an air bubble covered in mucus. The male does this over and over for hours until the nest is the size and shape that he wants, which can vary from fish to fish.

    Filling the Nest

    • When the bubble nest is completed, the female fish doesn't just swim up and deposit her eggs. Typically the male fish has to entice her over, or even take her there by force, as is the case with betta fish. There's physical contact, but the male doesn't fertilize the eggs until after the female has ejected them. As the fertilized eggs fall to the bottom of the bowl, the male fish scoops them up in his mouth and carries them to the nest, where he places them until they hatch, about two days later.

    Daddy is the Caregiver

    • Judith Wels's book "Do Fish Sleep?: Fascinating Answers to Questions about Fishes" says that for fish who actually care for their young, it's the daddies who are the caregivers, as with catfish and bettas. The males stay with the babies, also called fry, until they're free-swimming. That doesn't take long, only about four days after hatching. But during that time the male fish will watch out for the youngsters, warning them of threats by shaking his fins at the surface of the water to create waves. He'll also pick the babies up in his mouth to carry them back to the safety and comfort of the bubble nest.

    A Matter of Instinct

    • You boy fish may build a bubble nest even when there's no girl fish in sight. It's nothing to be concerned about, though. It's your fish's instinct to build a nest when he's ready to breed. Also keep in mind that a fish who isn't healthy and happy won't build a bubble nest. So the fact that yours does, even without the presence of a female, indicates that he's healthy, stress-free and ready to start a family.