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.
Make sure the place you are keeping the seeds and hopeful seedlings are in a place with moderate temperatures, not too hot or too cold.
Use nutrient rich soil, a fresh two-fistfuls of potting soil is what I always use.
Do Not Over Water. The pots should never sit in standing water, and should not water until the soil actually starts to dry out.
Rotate the tray every day. Depending on where you are keeping them, you need to give them ample exposure to the sun.
Make Sure to Thin Out. Sometimes we drop one too many seeds in the seedling space and you need to give ample room for things to grow.
Seedling need about 16 hours of light a day so make sure you have it in the sunshine near constantly or bring in a growing lamp to assist you in that. Even having a desk lamp with a fluorescent tube light would do.
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.