At 19 days rooted, they are only 2 days away from your schedule of giving nutes every three weeks, which is 21 days. Many times when a plant appears pale green with perhaps yellowing of lower leaves, it usually means they need some fert. It isn't throwing ferts at a plant if the assessment is good and you don't give too much fert. Your mix of ferts should be great for this grow. Fill a container with water first, then throw in the ferts in measured amounts. That way they get diluted right away and you don't have elements binding to each other. That would make them useless.