I think you can start just whenever the upward growth has stopped... But as I remember start very light with it.. I think I only started with like a couple of ml in a 15L res... & worked it up to about 7-8ml... But I was using far too much other shit as well so if your just using that & carbo as boosters you could maybe use some more... As I said if your flowers are fox tailing... Your using too much..