What paper weight should I look for in a bullet journal?

Across the specialist communities we read (r/bujo, r/bulletjournal, r/fountainpens), 100 gsm is considered the workable minimum for fountain pens and brush pens, 120 gsm is the comfortable sweet spot, and 160 gsm is what watercolor-heavy spreads demand. Below 100 gsm, ghosting and bleed-through complaints become common in reviews.