Hmm I'm gonna say its a ph/Nitrogen problem. Too much nitrogen will make the tips curl down like that. ph for soil should be 6.5-6.8 and soilless should be 5.8-6.2. What's your temps and humidity like in there?
Cool just fix the ph a bit and that will help. Temps sound good, you always want it in the 21-29C range. 40% humidity is great too with the size of those plants
PH generally drifts up over time. 6.0 with a few days of PH drift between watering actually seems pretty good.
I did a test of my water and measured the PH drift. It drifted about .15/day. So your 6.0 water was probably around 6.5 at your next watering.
PH drift is actually a good thing. At 6.4 you get the tail end of a couple of nutrients (cal and mag most notably). If you drift from 6.0 to 6.6 you pick up a better range of those secondary nutrients.
Check out the nutrient uptake chart and you'll see what I'm blabbering about....