How is calligraphy ink different from fountain pen ink? And what should I use?

Fountain pen inks are generally formulated to be thinner and water‑based, ensuring smooth flow through the fine channels of a fountain pen nib. In contrast, calligraphy inks are typically thicker and more heavily pigmented, designed to produce bold, dramatic strokes on dip pens and calligraphy tools. Keep in mind, calligraphy ink has been specifically desinged to be used with dip pens while fountain pen ink has been designed to be used in fountain pens. Using calligraphy ink in fountain pens can lead to clogging issues and may cause damage to the pen. Using fountain pen ink with metal nib dip pens can cause frustration because the ink is thinner and runs off the metal nib to quickly. You can thicken fountain pen ink with gum arabic to make it dip pen friendly, but you should no longer use the ink in fountain pens. The only exception to this is dip pens with glass nibs. The glass nibs are designed in a way where you can use either calligraphy or fountain pen ink with them.