White widow

If the picture shows the space you want to fill with a fully grown plant, then right now is a fine time to flip to 12/12. When an area is roughly half-filled, it's time. Yes, 38 liters is a big pot. you need about 4L / month and that errs on slightly large size. is it slowing down, or is growth more distributed? It's tougher to ssee when it is spread out vs top colas racing an inch or more per day upward. Did they get taller and you didn't raise the light to match? This can slow down vertical growth, if so. Top leaves look light green, but more than one thing can be the cause of that than just some minimal cannibalization from slightly too much light. if related to nutes, that could slow growth a bit too.

The pot size is enough. I grow in 12L pots. I would recommend to let it flower already, since it will probably double its size when stretching. Best of Luck! 🌱💚

Iron def, really common,if you are giving the correct amount of nutrients and your PH is ok look for a Fe chelate (edhha o-o if possible because it works at wide ph range), you can spray it or give it trough roots, the 2nd one is obviusly slower, if you want to see a fast recovery spray them and if you see that they recover quick add a little amount 2 weeks after trough roots to enhace de availability trough the rest of the grow. Always follow the instructions prior use and adjust the amount to the grade of deficiency you have usually the dosages are greater for foilar application and much fewer for root application so keep that in mind.