the single leaf doesn't mean anything, I remove it as soon as the others cover it
the other leaves are yellow because being underneath and doing reduced photosynthesis, as soon as the plant needs a little more nutrients and doesn't find it in the soil, it goes to look for it in its reserves
which are indeed the leaves
if you're using 1 ml/l of biogrow, you're using half of the recommended amount. I suggest you increase it because the plants are starting to get big, and the bigger they get, the more they eat