Leaf tips curling ?

I'm not sure what is going here, i do not beleive its overfeeding or heat stress, im more inclined to think it's a humidity issue as my humidity does not go above 30% (waiting on a humidifier) would low humidity levels cause leaf tips to curl down?

Running a 400w HPS i beleive i noticed this issue a week after swapping from a LED (veg) to the HPS
6.0 ph balance.
In COCO
Had a mite problem but thats been solved, plants look overall healthy other then the leaf tips.
I water the plants every 2-3 days ATM, or when i notice top soil completely dry.
In 2nd week of flower.

low humidity won't do that...changing from led to hps can mess with them until they get used to it but it looks more like overnute or heat stress to me