Answer: The young zebrafish has a completely transparent nervous system, making them ideal for studying development.
For about a week after they are born, zebrafish are transparent under high intensity light. This allows an experimenter to identify individual cells of the nervous system.
The males usually fertilize the eggs externally, which makes them easy to harvest. Furthermore, these embryos are often large, scientists can easily modify their DNA using microinjection directly into the young cells.
Additionally, it is feasible to collect several thousand embryos a day, giving statistical power to experiments. An added bonus is how many zebrafish can be kept in a fairly small space compared to rodents, for which housing is more difficult and costly.
You can read the details of the molecular experiments that are used to study zebrafish here:
