Weird spots on lower leaves spreading quickly

[NooberMcgee]

Hey Y'all! First time grower here needing some much needed urgent help for my girls.

Lights: 600W MH through Veg then changing to HPS. 24/7
Age: Almost a month old.
Medium: Happy Frog with a little extra perlite for drainage.
Container: 5 gallon pots transplanted from solo cups.
FIMd and LST'd
Temp: 68-78f depending on weather. (grown in my garage)
Grow room: 4x4x8
Nutrients: I just started the Grow Big/Big Bloom at 1/3 strength but only after I started seeing signs of spots.
Water: Distilled.

At first I thought it was pests although I've kept the grow room pretty sterile and away from the outside world but I cannot locate a single one. Carefully searched all leaves under and over and the soil. Started a neem spray schedule just in case.

PLEASE HELP!! Heading out of town for a few days this weekend and would really like to nip this one in the butt before I go.

 

[ThcSnow]

Hey man are you giveing them any cal mag suppliments? Distilled water is devoid of heavy metals and such since its pure evaporated water!

 

[OEF Vet 11B]

I second cal mag. Saw earlier in another post looking similar and cal mag was suggested.