Nest building fish are those that build nests for their eggs and usually one or both parents will watch over them. These eggs are well protected, and it is important not to disturb the nests once they are built.
Bubble nests are perhaps the most fascinating types of nests. They are often associated with Bettas, though some other species will build them as well. To make a bubble nest, the male fish will gulp air and blow it out as bubbles which are affixed to each other with saliva, creating a floating mass which holds the eggs and later the fry up near the surface. The female will then lay the eggs, and the male will put them in the nest. If a baby fish escapes, the parent will scoop it up with his or her mouth and gently deposit it back into the nest.
These bubble nests will break up if touched or even if the water is disturbed, so great care is needed. Often, only one parent will guard the bubble nest, so you can remove the other fish to ensure quiet water.
The other type of nest builders will use plants or existing caves to lay their eggs in. Some types of cichlids, for example, will lay and fertilize their eggs in a hole or cave and keep watch over the eggs until they hatch. They are usually fiercely protective of their young, but you can ensure the safety of the nest by removing any other fish that are in the tank. These parents will probably not eat the babies, but other adult fish may see them as appetizing.
It's important to understand how your fish breed so you can look after them properly. You'll understand the signs and know when to start watching for babies. Whether you breed for fun or money, it's a fascinating process.