(Source: M. Westerfield from Zebrafish Book 5th Edition)
Zebrafish are photoperiodic in their breeding, and produce embryos every morning, shortly after sunrise. For continuous production of a relatively small number of embryos (30-50 per tank per day), use an equal number of males (longer, slimmer, and more yellow especially on the belly) and females (plumper and more silvery). Control the day-night light cycle with an automatic timer (14 hour light/10 hour dark) and keep the fish well fed with a rich supply of food like liver paste and live brine shrimp (see Food). If the tank is in a room that must be used during the "night", cover it with a black plastic drape to keep out ambient light and provide a separate tank light, inside the drape, that is controlled by the automatic timer. Be sure to check the temperature of the water with the tank covered; you may have to adjust the heater to compensate for the light. Cover the entire bottom of the tank with a single layer of marbles to keep the fish from eating the newly spawned eggs. Each morning, siphon the bottom of the tank for embryos as described in Embryo Collection. If you do not need embryos for a time, remove the marbles to let the fish "rest" and feed much less food. Otherwise, the fish will get fat and breed poorly.