(Source: M. Westerfield from Zebrafish Book 5th Edition)
Live zebrafish embryos can be viewed at low magnification by placing them in depression slides filled with embryo medium. After 16 hpf of development, they become spontaneously motile and can be anesthetized with tricaine (see RECIPES, Chapter 10). After 23 hpf of development, they begin to develop pigmentation that can interfere with some observations. To maintain optical transparency grow the embryos continuously, starting around 15 hpf, in 1-phenyl-2-thiourea (0.2 mM in embryo medium).
For observations at accurately determined developmental stages, it is imperative to maintain the embryos at 28.5°C at all times. This can be accomplished by keeping the embryos in small Petri dishes or covered beakers on a microscope slide warmer and by using a microscope stage warmer, or alternatively by maintaining the observation room at the appropriate temperature.