Every aquarist has his or her own tricks or preferred way to hatch out eggs, whether from annual or non-annual species. What follows are a few of the tips gleaned from the extensive experience of a number of killikeepers.
An egg is usually ready to hatch when the iris of the eye is distinct and shiny. If a light is shone on the egg, the heart will begin to beat, and the embryo will begin to move around inside the egg. However, that doesn’t necessarily mean that the embryo will hatch.
Fish eggs are surrounded by a protein coating known as the chorion. This is exceptionally thick in killifish. When the embryo is ready to hatch it produces an enzyme, chorionase, that helps to break down the chorion, allowing the fry to more easily break through the “shell.”
One thing that helps trigger production of chorionase is CO2, hence the success of those force hatching methods which increase the CO2 level in the hatching container. Exhaled air contains 90-100 times more CO2 than does the atmosphere, thus blowing into a hatching container often speeds up hatching. Some aquarists place “reluctant” eggs into a plastic 35mm film can, blow into it, then carry it around in a pocket for a few hours. This combination of added CO2 and agitation has resulted in nearly 100% hatches.
Another method that has proven successful is to make a complete water change on eggs that are ready to hatch, using water that is 10warmer. Warmer water contains less dissolved gases than colder, so the lower CO2 levels come into play with this method, as well. Adding microworms or a small pinch of yeast to the hatching container has the added effect of providing food for the newly hatched fry. GCKA Newsletter, April 1997