Many species of labyrinth fish, including Bettas and Gouramis build bubble nests to incubate their eggs.
A bubble nest consists of saliva coated bubbles which are constructed near weeds and branches. They are constructed by the male, which is also responsible for guarding the eggs and young.
During the spawning, the male will capture any eggs and deposit them into the nest. When the eggs hatch, the male will actively catch any fry that fall out of the nest and place them back. During this period, he will aggressively defend the nest against any potential predator.
Bubble nests are mainly built by fish species that inhabit stagnant, oxygen depleted waters. By being close to the surface, the nests ensure that the eggs and fry gain enough oxygen to allow for normal development.
(Three spot gourami, Trichopodus trichopterus depicted)
Alberto Garcia on Flickr