6ml is a little high but them plants dont look burnt for it, and ive known people in this thread use 7ml per ltr with no signs of burn n shit, i grow a hardy strain exo cheese n when in coco after also underfeeding a few coco grows and seeing that 7ml didnt burn em i use 5ml per ltr from the first week of flower til the end n they love it.
but yes i am saying just water more often, that coco looks dry as fuck n them plants a little underfed, and on 6ml per ltr unless ya not feeding them often enough theres no way they should look underfed on 6ml.
also yeah i get asking for advice n all that, but this prob aint ya best place most in here are quite confident in there growing yet many use different methods n ways, your get a shitload of cross advice which wont help, best advice would be to just read read n read some more n learn from ya own ,mistakes, yeah they look a lil underfed n the coco is dry but ya really not doing that bad they are decent looking plants.