My current favourite for medium lines in my Bullet Journal- Sakura Pigma Graphic! These Japanese import fine line pens have a firm nib to create 1mm strokes perfect for writing and drawing in your Bujo.

Each Sakura pen has black micro pigment ink that's fade and waterproof when dry. This again makes them excellent for journalling as the colour wont bleed or run when you use them alongside highlighters or other coloured pens.

A superb fine line pen- every pencil case should have one! This listing is for one pen- but you'll probably want a couple after you've tried them out! ;)