Leaf browning, diagnosis? Need help

Hey all, im in the middle of my first grow with some DWC autoflowers, the one plant that started flowering early has started to develop some brown spots over some of the leaves and i was hoping maybe some veteran growers could help me diagnose the problem so i can fix it.

It seems that its narrowed down to either cal/mag problems or (fungus?)

PPM ranges from 400-600 depending on how long since ive added more water,
I use Flora Micro 3 part nutrients and Hydrogaurd only right now.

Lights are probably about 12 inches from top of bud sites and 19-20 from canopy. (Also the leaves underneath the canopy are browning so i dont think its caused by the LED lights

I run an exhaust fan and an internal fan for air circulation with humidity ranges from 40-60%

The PH does slip down sometimes even below 5 sometimes when they drink alot of water throughout the day, but that is normally taken care of in less than 24 hours.

Thanks in advance for any input, and here are some photos from today (sorry for quality still use an old phone)

Calmag is a must in DWC I learned that on my first grow it also depends on what order you mix you nutes

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Yep cal mag u say ur ph drops below 5 . Both cal and mag need upper 6 to absorb, water a little high next time or 2, 6.7 6.8 I had little spots and they went away with 10 days

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Thanks You guys!

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