Bummer,well you'd think the soil is good for pH.Your water is bottled right?
What about light,I assume you pulled it back?
Im stumped ,fresh dirt should be happy as all get out.
A plant that size could be above 1EC 1.5 even.When I did hydro my pH was more stable when EC was higher.But I used Tap water around 200 ppm.
In your case Id flush it like feeding in coco,Pour from the top and wash it out with some food.
The system you have is just like ebb and flow isnt it?