Any aquaponics system starts with a good seedling. As you probably know growing in the expanded shale or lava rock is hard for a poor little seed to do, you need to first get them started in some seedling beds by doing the below number of actions.
Plan on nurturing these little buggers for 5 or 6 weeks before you plant them into your chosen growth medium in the closed system.
Make sure you are getting the right seeds you need to have a robust growth period. Because you will be keeping these plants around a bit longer than you would in planting them in an outdoor garden, you need to make sure that the seeds can hold up to all that work. Do your homework on the seeds you get for the system.