Many aquarium fish, including mollies, guppies and swordtails, bear live young but most species lay eggs. Spawning strategies to ensure that at least some eggs survive predators long enough to hatch vary widely. Some fish count on safety in numbers, releasing so many eggs that some are bound to hatch even without parental protection. Others produce fewer eggs but provide more security by making them more difficult for predators to find. Still others go to great lengths to conceal and protect their eggs by building nests or holding the eggs in their mouth until they hatch.
The Scattershot Approach
Female egg scatterers such as tetras release large numbers of eggs into the water and leave the rest to chance. Fertilization takes place when the eggs come into contact with the milt, or semen, released by a male into the same area. Adult fish, including the parents, often eat these eggs before they have a chance to hatch.
Egg Depositors and Buriers
Female depositors such as some cichlid species lay sticky eggs on plants or on the surface of the substrate, where they're fertilized by males. Egg buriers such as killifish dive down into the substrate to release their eggs. Males follow to fertilize them.
Mouthbrooders and Nest Builders
When mouthbrooders such as the chocolate gourami and African lake cichlid take fertilized eggs into their mouths to prevent them from being eaten by other fish. Some species, including Siamese fighting fish, use their saliva to build bubble nests for their eggs. Other species collect plant materials to build nests or dig nests into the substrate.