Kaweco inks are one of our favorite options for fountain pen users. The colors are vibrant and saturated, with visible differences between thick and thin areas for a textured, handwritten look. It resists bleeding and feathering on suitable paper (we suggest Rhodia or Midori paper for fountain pens), though it is not waterproof.
Kaweco ink cartridges are international short size, so they will work with other pens that use the international short format. They will not work with brands like Lamy and Pilot, which use proprietary formats. Of course, they work with all Kaweco pens. For other brands, we recommend that you check your specific pen and make sure it's compatible. Six cartridges to a box. Made in Germany.