These girls are 6 weeks old, and in 2 gallons of FFOF. They are primarily outside, and we're topped a week ago.
I noticed the rust/brown spots on one yesterday, and
I’m pretty sure I see tiny white spots on your leaves. Check for bugs cause I had the same issue and it showed like that cause bugs were attacking my roots before I even knew it.
High feeding plants need cal-mag no matter the medium. Depending on your water would determine how much in soil. You can amend it in or water it in but it needs to b there..