oh, definiteyl start a fertilization regimen. seed will power the growth for 2 weeks give or take, but a safely fertile substrate ready to provide is a good thing to have before the seed runs out of power... the cotyledons will fade a bit as first sign it needing more. could wait for that and pre-empt it next time (assuming you stick to same products)
i'm not familiar with "light mix" is that soilless or amendments come with it? if not soilles, do the typical ramp up.. start slow, increase concentration to full strength over time. if soilless, you want to feed a 650-750ppm well-balanced mix of nutes every irrigation. you may exceed that in a soil grow, but you also probably won't feed every irrigation in that context, so it ends up being the same rate over time that the plant can handle... has to be.