Hi. I also had a similiar set up as you. I used the same soil. Same foil. Same problems. My plant also looked very similiar to yours.

i had that burn shit on my plant also. it started on the lowest leaves and slowly worked its way up to the other broad leaves. I wasnt able to stop it. and no one ever told me what it was. It may be the soil... but i dont really know. The problem happened when they hit bloom. Hopefully it wont happen to you.
Flower now. when they start forming buds. just tie back the bigger leaves that are blocking light from getting to the newer buds/growth. thats LSTing. Using small string,yarn, twine, whatever... is good. anchor it to a rock or something heavy tho.
maybe get a couple CFL and throw em in there to help you out. Your definitely gonna need more lights. Also be prepared if your plants get to 4ft high. flo lights sometimes cause the plants to stretch to get really tall, so just be prepared for it. A 12 in tall plant in Veg turned into a 4 ft one in bloom. goodluck.