The problem started two weeks ago after I realized the soil pH of my Mental Floss was 3.2
I flushed it until it was back to 6.0 and the necrosis slowed down for a while.

Now, it's back in full force after I flushed the plant with mineral oil because I had 10 gnats flying around the plant.
The flush did kill all the gnats but now, the plant is dying at a rapid pace and i don't know what to do.

I tried flushing, correcting the pH, feeding and I even tried molasses.
Nothing works.

The worst part is, the exact same thing happened on my first grow and I ended up chopping the plant early because of it.
What do you guys think is going on here ?

What should I do to fix this or at the very least, slow down the issue for 2-3 weeks ?

1. Are you growing from seed or clones? Seed
2. How old are your plants? 2 months and 3 weeks
3. How tall are your plants? 1,4m - 1.8m
4. What size containers are they planted in? 3 gal
5. What is your soil mix?
6. How often do you water and what type of water do you use? every 3-4 days, tap water
7. What is the pH of your water? 6.0
8. What kind of fertilizer do you use and what is its NPK ratio? Maxibloom, 5-15-14
9. Do you foliar feed or spray your plants with anything?
10. What kind of lights do you use and how many watts combined? HPS, 400w
11. How close are your lights to the plants? 50 cm
12. What size is your grow space in square feet? 3x3
13. What is the temperature and humidity in your grow space? 76-68F, 42% humidity
14. What is the pH of the soil? 6.0
15. Have you noticed any insect activity in your grow space? Fungus Gnats
16. How much experience do you have growing? This is my second grow