Brown Spots and Purple leaf underside

Hi, I'm new to this forum.
On the whole my plants still look good.
But I'm starting to get brown spots and the underside of my leaves are turning purple.
The brown spots are only on the older leaves of the older plant.
The purple is faint except for the one in the picture.

I am growing in a closet with attic access and no source of heat other than the light and my lows outside at night are 65. So the purple could be from low Temps at night from the attic. Can I get a definitive answer? Thanks.

As far as the brown spots go, I'm not sure. My best guess is a calcium deficiency but typically that's newer growth and you get yellowing too, which I have none of.

My leaf stems look a little pink from a corrected nitrogen deficiency but the new ones are green.

I have a 1200watt LED 12" from plants
I use a deep water res with airstones.
My ph is between 5.85 and 5.9
My PPM is around 850

Per gallon
Blue Planet Farmer's Pride
10mls of all 3 mixes
Blue Planet Liquid Seaweed
5mls
True Plant Science CalMag OAC
2mls

This is my first grow in 2 years so I'm a little rusty. Normally, I'm good enough to avoid plant sickness, but I'm having a little trouble. If someone could help me that would be awesome. Thanks.

 

I don't think it's root rot because the organic nutrients I use are brown and black. So they stain every thing they touch including the rock wool.