Any changes to the water source? When I switched from buying spring water to having my own RO filter, my plants all freaked out many looked like that, mine turned out to be a cal/mag issue.... and do keep in mind 6.0-6.7 is still a pretty wide window of Ph... try throwing a handful of EWC on them for the next few waterings, see if they perk up.... Not saying its the problem, just think all plants really fucking like some castings, lol.
Im assuming you checked all the simple shit, air temp/humidity and made sure its not bugs, so I skipped those.