Help Quick brown spotting lower fan leaves spreading fast - I have pictures

Help -Big Bud x Northern Lights brown spotting lower fan leaves
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I am am growing Big Bud x Northern Lights hybrid.
I just noticed that the lower couple sets of leaves have a brown / reddish spotting it has spread fast and I have no clue what it might be. The rest of the plant seems healthy and has been growing great. The new growth is green but the spotting is going up the plant and turning it brown. I have seen a couple gnats but nothing to bad.

- I am 3 ½ weeks into flowering at 12/12 light cycle
-using fox farms ocean forest soil in 11” pot
- 150 watt HPS
- I do every other watering with Fox Farms Big Bloom and Tiger Bloom solution (I use half recommended strength)
-Case temperature is at 80 degree with lights on and 74 degrees without.
- I use purified bottled water.

my grow methods are very similar to yours, in my experience leaves browning or yellowing or have necrotic (dead) patches, especially around the edges of the leaf, which might be curled. the plant may be too tall and or developed a Potassium (K) deficiency.

Do you check the PH of your water & the runoff? I trace almost every problem back to PH in some way it seems...

Also, I think if you're paying for water, get distilled...the purified stuff may still have minerals in it.