Got something called Pumice stone at Dollarama but it feels like plastic and make PPM go up pretty fast so I'm questioning what this product is and why we can't find the good growing supplies in Canada.
For sure, learn the plant...there's a trend with these posts recently of extreme defoliation coincides with knowledge being around of making the plant produce better in terms of medicine and to save water and nutes. Something about cell being sent out to repair translate to you...
This heavy defoliation will use less nutes and less water so easier to manage his res going forward. And from what I can understand the cell growth will have better medicinal benefits.
Any update on how you manage the aciditiy? Everything I read points to the presence of phosphorous is good organic soil and its release manages acidity where other acids or related compounds are present but are snake oil if used excessively.
Look into rockwool and EC from the manufacturer and it has to stay wet but you may get algae on top depending on a few factors.
Seems like your water is probably chlorinated and the alkalinity is giving you EC control issues.