Your media mostly. I planted seeds in my bed and the only ones I did not count were some heirloom lettuce. Way to small for my eyes so I poured in a general vicinity. Currently have 18 sprouts growing, planted 2 bush bean seeds one survived the other sprouted and died. Planted 2 Portugeese Chili seeds and two are growing. Planted 8 Cherry Tomatoes and 2 are growing 1 more just sprouted.
So I guess lettuce would be the one too hard to keep track of. I used hydrocorn and made sure to plant the seeds at the right depth. Lettuce sprouted in 3 days, others all took about a week. The seeds settled in the cracks of the hydrocorn. Also I changed the depth of the water in the bed to have it fill right below the top of the media, and the flow is so gentle, even on the drain that I suspect I have not lost many if any seeds to the hungry fish.