Newb question on using nutrients in Rock wool Cube seedlings

I currently have 3 seedling in rock wool cubes under 125 watts CFL's on a 18/6 rotation one of the plants has been germinating since June 22nd so its been growing 17 days the other two have been germinating since June 27th 12days. The 17 day old plant is growing nicely and it already has its second set of leaves I want to know when to start using nutrients on this specific plant? I was told to start watering them with nutes as soon as the second set of leaves starts showing. I was gonna mix Thrive B-1 Green, Sugar Daddy, Root 66... I have a picture at the bottom.

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slap her with a drop of superthrive. she is too small for a full doze of those nutes yet

No nutes until the Cotyledon starts to yellow. Then you go small, 1/4 dose or so.

The Cotyl is basically your litmus paper for when the plant needs help. Until it changes it's going to use up the given nutrients from being a seed.