There wasn’t much about the success rate of starting off seedlings in clay pebbles in any of the aquaponics resources I researched, but I thought I would give it a go as transplanting a clay pebble pot straight into an aquaponics grow bed is much less disruptive to the plants than washing soil off the roots and redistributing them in different media.
Whilst I was waiting for my aquaponics system to properly cycle I started off some spinach, pak choi and basil, in the same clay media as my grow bed. I also grew some seeds in soil to monitor the difference. Obviously as the soil has more nutrients after about 10 days the soil plants started to grow much faster, however once in the system the plants in the pebbles were eventually much stronger than the soil plants. I think having all the roots washed and rearranged in pebbles did not leave the plants in a happy mood and in-fact some of the wilted and perished! This did not happen with any of my pebble seedlings.
VERDICT: Start off your seedlings in the same growing media as your system. Plant them in a pot with holes in the bottom so you can pop them straight in your grow bed. Grow them for about two weeks (or until the roots poke out the pot) before putting them in the main bed, and if possible water them with the same water that is in your system BUT only if it is fully cycled.
TIP: A heated seed tray helps development and of course lots of light is beneficial.