I've been researching this problem and sometimes I'm convinced it's Mg deficiency and sometimes I think it's Cal deficiency and sometimes I think it's K deficiency!
What do you think it is?
Bubble bucket - city water adjusted PH 6.8 - GH products and schedule.
OP, definitely drop the ph to 5.8-5.9 and see if things change over the next couple of days.
Here's a basic chart I found online, notice the hydro chart on the left? If the plant is OG then definitely keep her in the lower spectrum of the 5.5-5.9 ph levels. If not an OG then 5.8-5.9 should be fine.