Curious, i was having a similar issue my last run. What i found was that the soil, well root ball wasnt getting to dry out enough causeing slight bactirial, fungal, algea issues inside the soil. Everything looks good, PH, PPM, Smells,
Everything. But the leaves had the edges fertburnd looking with them darn copper spots. Never really got it worked out or found a definative answer to the issues, all i can do is tell u what i did, i let em dry up till they were wilting slightly then watered. I also had a minor Gnat issue too, but that is sorta what i saw when that all started for me. Sucks cause honestly i never really figuared that shit out.
All things considered though, they look great.
Best of Luck!
Peace,
S i R