The short answer is: yes!
However, all inks vary slightly in colour intensity and flow characteristics so not all inks are necessary ideal for all pens but you will not cause any damage to your pen.
Important note: only use fountain pen ink, not any other type of ink. For example, calligraphy inks usually contain pigment solids and resins which will dry up and ruin your pen! The same is true of India ink. If in doubt, use only the inks that we stock.
Historically, fountain pen manufacturers insist that you use their own brand ink. There are two reasons for this: firstly, it helps them sell more ink. Secondly, it ensures that the user only buys ink suitable for the pen (in other words, not calligraphy ink or some other ink that will certainly clog up the pen). There is no need to just use the manufacturer's own brand ink in your pen.
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