We meet again, Mr. Farts.
Dump the res IMMEDIATELY. A cup of epsom salts is way too much (unless your res is the size of a small town water supply cistern) and you may already have caused irreversible damage.
Is this the first time you have run this strain in silica stones, and are these stones designed for growing cannabis in hydro? I am concerned with additional minerals leaching off the medium (acid solutions leach off mineral and metal substances faster than neutral ones) into your ALREADY hard water.
Would you happen to have a water report on your hard water? What is it that contributes the hardness, CaCO3, sulfur, iron, ....?
I'm going to suggest switching to a 'hard water formula' hydroponic nute system IMMEDIATELY and save the home-mixed nutes for the more forgiving medium of soil, where it has been working for you. I fear the ppm of your solution, while it looks REALLY low for the Cheese (Skunks feed heartily, if not hoggishly), that ppm may be contributed by 'junk' components that your plant can't use or doesn't like. That plant looks starved but suffering and in absence of the tip-burn associated with overfert, I think it's the formulation of your nute system and the hardness of your water and a possible interaction with the medium.
Just out of curiosity- ask your friend if he/she adds a significant level of organic acids to that system.