Extremely clean water. Most fish will not breed if there is any ammonia or nitrite present, and large amounts of nitrate are toxic to baby fish. Some fish, especially tetras, must be bred in a breeding tank that is bare and sterile so that their eggs do not fungus. A varied diet. Fish that are producing eggs need better food that fish that are just living in a community.
Breeders call the process of specially feeding parents conditioning.
Conditioning foods include live foods, fresh frozen foods, or spirulina based foods. Find out the specific requirements of the fish you intend to breed. The correct environment. Fish that breed on substrates need proper substrates to breed on, like peat, rocks, shells, or plants. Some fish are shy and require a lot of cover, caves, or dim light.