Bubble nest

Bubble nests, also called foam nests, are created by some fish and frog species as floating masses of bubbles blown with an oral secretion, saliva bubbles, and occasionally aquatic plants, or an area for egg deposit attached at the bottom. Fish that build and guard bubble nests are known as aphrophils. Aphrophils include gouramis (including Betta species) and the synbranchid eel Monopterus alba in Asia, Ctenopoma (Anabantidae), Polycentropsis (Nandidae), and Hepsetus odoe (the only member of Hepsetidae) in Africa, and callichthyines and the electric eel in South America. Most, if not all, fish that construct floating bubble nests live in tropical, oxygen-depleted standing waters.Anabantidae are the most commonly recognized family of bubble nest makers. The nests are constructed as a place for fertilized eggs to be deposited while incubating and guarded by the male until the fry hatch.

Bubble nests are built even when not in presence of female or fry (though often a female swimming past will trigger the frantic construction of the nest). Males will build bubble nests of various sizes and thicknesses, depending on the male's territory and personality. Some males build constantly, some occasionally, some when introduced to a female and some do not even begin until after spawning. Some nests will be large, some small, some thick.

Various stimuli have been shown to onset the construction of Bubble Nests, such as rapid changes in temperature, barometric changes, fluctuations in rainfall, various fish tank materials, and presence of other males or females.

...
Wikipedia