Isolation of Zebrafish embryos Zebrafish will mate and deposit fertilized eggs on the bottom of the tank at 'dawn'. They can be accustomed to lay at any convenient time by keeping the room dark and lighting the tank with a timer. The maximum embryo production is obtained with a ratio of 1 male for every 2 females, with a group of 20-25 fish. The night before embryo collection, vacuum the tank well with a siphon to remove debris several hours after the last feeding. Deposit a layer of washed marbles to cover the entire bottom surface of the tank. Do not disturb for the first 30 minutes after 'dawn' to allow fish to mate (see below). Embryos can be collected once a week, with best results from tanks kept on a regular schedule. Collect embryos and fertilized eggs from between the marbles with a siphon and collect with a fine fry net or mesh filter. Transfer the net to a glass dish full of tank water and examine the embryos with a dissecting microscope at low magnification. Transfer healthy embryos to a clean glass petri dish containing embryo media with large-bore glass pasteur pipets. Discard any that are cloudy or ruptured. Keep embryos at 28°C. Zebrafish mating behavior.