Curled tips on leaves, what's the cause?

hi there Thano, this is a typical sign of root zone issues. This is a because the "grow big" contains quite a lot of ingredients that promote soil life which you usually want to cut out as you reach flowering. For example Kelp and earthworm casings which can promote mold for example. the good news is you identified it early so it's not too late to cut out the grow big. You want to fix the root zone issue by breaking down the problem roots using something like beneficial bacteria like bacilus subtilis and mycorrhiza or better yet humic acid. If you do this the problem will stop spreading almost immediately and the the plant will recover fully within five to ten days. You can tell it's not an NPK issue because you're adding plenty of PK and the plant is a dark green color meaning there's plenty of N. Anyway , hope this helps ! 🚀