Ok, as promised, here is an update!
They complete today 5 weeks from seed and I'm quite happy as they grew a lot more this week than in any other.
Two of the photos show some white salty-looking crusts that I've been finding on the top of the soil. What is this? Salt build-up because of the nutes? Should I flush them more? My watering schedule is something like this: One day I water them with Bio Grow and Hesi Super Vit, next day plain water or with Super Vit only, next day, if the soil is still moist I do not water them, if it is dry I water them again with nutes. Bio Grow states 2 to 4mL per L of water. I started with 2 or even less, now I'm giving 3mL per L and I hope to give them 4mL this week (before going into flowering). I still don't know if I'm going to change them to 12/12 at the end of the week or at the end of the month. They are about 10 inches (most advanced) and 8 (least advanced) high. I know they could veg a lot more but the grow box is not really high and if they double size during flowering, I'm afraid they might get too close to the 400w HPS.
Any suggestions?
PS: the pics where taken with HPS turned on.