you can start soon after they root. just make sure you start off with very little and build your way up. lets say the bottle says 10ml a gallon, do 1ml, 2ml, 3ml... thats my way works for me. what i have noticed is plants just love slow changes. and babies including humans love to start out with calcium. i don't know why i even said that but its kinda cool. thats in just about any fertilizer and its def in water.
just start out slow thats all.