Plant spawning killies such as those of genus Aphyosemion or genus Epiplatys lay their adhesive eggs singly among the fine leaves and roots of weeds in their natural habitats. In the aquarium, they may simply be left to procreate among the leaves of a densely planted aquarium (Java Moss is especially useful for this purpose), but this strategy leaves eggs and new fry at the mercy of their parents and other tank inhabitants, and vulnerable to the attacks of fungus. This method does often work, but other more productive methods have been developed.
Since they are so tough (and often large), killifish eggs can be easily collected by hand and removed to safer premises for incubation. To make the eggs easy to find, you provide a floating nylon yarn mop as a substitute for natural plants. It's easy to make a mop. First, wind a length of colorfast yarn (color is unimportant, but I use brown because it makes the eggs easier to spot) around a book about 8 or 9 inches (20 to 23 cm) high. Make about 20 loops. Then slip a shorter piece of yarn (5 inches or 13 cm) under the winding and knot it tightly around the bundle at one edge of the book. Finally, cut through the bundle at the the opposite edge. You can tie the ends of the knotted yarn around a cork if you wish the mop to float, or leave it as it is and let it sink. It's a good idea to wash the mop in hot water (with no soap) before using it.
When your killies have spawned, pull the mop out of the tank and squeeze it gently to wring out the excess water. Then, working in moderate light (some eggs may be harmed by exposure to intense light), examine the mop. Fertile eggs look like tiny crystal balls, transparent and shiny. You can remove them from the mop with your fingers, a toothpick, or tweezers. Within a short time of being laid, fertile eggs become quite hard, so you don't need to worry much about crushing them. I use a toothpick (because I have clumsy fingers and don't trust myself with tweezers), which I place next to an egg and rotate. The sticky thread that binds the egg to the mop adheres to the toothpick; when the toothpick is removed the egg comes along with it.
Killie keepers incubate the plant spawner eggs in several different ways. Some store them in a container of water with acriflavine added to inhibit fungus growth. I use one drop per cup (240 ml) of water. As the eggs develop, some inavariably die or are infertile. These turn white and may become covered with hairy fungus, and should be removed lest they infect the viable eggs. It's a good idea to move the eggs daily to a clean container (washed without soap) with a fresh acriflavine solution; this helps retard fungus growth further. Fertile, healthy eggs gradually start showing pigmentation and soon eyeballs are apparent. When the egg stares back at you (that is, when the iris is well-formed and shiny) it's ready to hatch. Depending on the species and temperature, this may take between 10 and 28 days.
The dry incubation method sometimes works when the wet method fails. Harvested eggs are placed singly atop moist peat moss in a closed, darkened container. You place them apart from each other to prevent fungus from spreading among them, and check them every day to remove dead and fungused eggs. When they are ready to hatch, you place them in some tank water and start preparing some live food for them to eat. This method is sometimes preferable because it seems to reduce fungus attacks on the eggs and allows you to coordinate (within limits) the hatch times of multiple batches of eggs.
Killifish eggs sometimes suffer from a strange condition. Although they may be obviously alive and ready to hatch, some never seem to able to break out of the shell on their own. These fry are likely to die in their shells unless they are helped along. Hobbyists accomplish this by increasing the temperature, dissolved carbon dioxide level, and movement of the water. That is, they put the eggs in a vial with a little water, breathe into the vial and cap it, and then carry the vial around in a pocket, shaking it occasionally. Believe it or not, this often does the trick. Others add a few microworms (nematodes) to the egg container to boost the CO2 level and provide some agitation.