Time to use nutrients ?

Yes, feed from day 1 in coco,from first watering.Cotyledons can handle the first week but it's not a good way to reach that stage,they'll be dead from then.We don't want our cotyledons dead,instead you can keep them OK for a healthy grow.Plant's natural energy shouldn't be wasted at this critical stage,my two cents.It'll need it later.Soil naturally has microorganisms and elements which can carry your plant to first and second set of leaves,maybe more.But coco can't.Even in soil,you don't let your cotyledons do the job.It's the nutes in soil that push the seedling for a couple of weeks.don't put the pressure on tiny little cotyledons.it's the last source of energy,in case of emergency for a seedling until it settles roots.if they are pale,yellowish,it means it is hungry and will cause growth problems.especially if you are using strong lights.Let me show you some of fellow growers problems : growdiaries.com/grow-questions/34550-help growdiaries.com/diaries/106660-grow-journal-by-dorweed/week/560322 growdiaries.com/grow-questions/33266-why-are-my-autos-so-small-its-week-3-and-im-scared wish 1st and 3rd ones had a diary but for the 2nd one,you can see the development after introduction to feeding. that plant didn't even make it to 2nd set of leaves in a healthy way,it looked terribly pale because of lack of energy.you don't want your seedling to look like that.it is simply when you let your cotyledons push the seedling to that stage.coco is inert,strictly needing to be fertilized.unlike soil. I just want to share my own coco experiences with people who are new to coco.that's all. start with 0.5ml/lt of base NPK nute and half or full dose of calmag supplement.you'll need silica as well,rhino skin by advanced nutrients works good.But don't use it in flowering,only until the first half of stretch (about 2 weeks into flowering and stop) Okay,this was for coco. Your growing media doesn't look like coco to me. Are you sure it is coco? It looks very soil from here.