Update:
These past few days I have been experimenting with the distance between Lights and Canopy. On Monday, after adding exhaust fan to my tent, I put my lights at the max distance I could - 23'', everything seemed to be ok. but after 24h there was no progress in growth, not even stretching.
So yesterday I decided to move the a little closer and see what happens - 20'' from canopy, still no progress in new growth but newest fan leaves kinda grew bigger, but the old ones started displaying few brown spots. This is where I started extended reading up about pH issues and different nutrient deficiencies lockouts and other problems. And for a moment there I thought this distance is just fine and I can leave it like this. But then I quickly realised, after reading other LED journals, that a lot of growers keep more powerful panels a lot closer then 20''
Today I lowered lights even more - 17''from canopy, then suddenly after 3 hours I noticed new nodes growing and new leaves opening, soil getting drier deeper in, but then again new brown spots showing up not only on older fan leaves but on newer ones as well and some yellowing appearing. I'm almost sure I need to flush her with pH'ed or weak nutrient pH'ed water tomorrow morning when watering, most likely, will be due and see if situation changes. I know its not heat stress or hotspots because fan is blowing directly on her and my other seedling does not show any other issues besides slow growth and stretching, strain should be very fast growing.
Here's a pick with brown spots and yellowing.