young leaves have light colour and growing slowly

first, make sure it's not a little interveinal wrinkle, as that can play tricks on the eye as well as pictures. I see a huge difference just from the grow lights to more natural lights. step up in there with that natural land and flatten a bit for a look. Also, new growth can grow out of it... if you eliminate all other possiblities it could be a S deficiency. If you use RO, could start using plain tap, but need to make sure pH stays same range. Ensure it isn't something simple before playing with balance of nutrients. look over what you feed, calculate ppm of mix etc etc... this can help deduce what the real problem is without guessing. 6.0-6.8 is a bit too large of a swing. If soil go for 6.5-7 or 6.5-6.8. If soilless, ~6.5ish and some sway is fine.