A part of the fountain pen writing experience is the type of ink that goes into the pen. Just like gasoline for your car, the better the ink the smoother the pen will perform. You’ll get out of the pen what you fill it up with.
A basic rule of thumb is that only things you should put in your fountain pen is fountain pen ink or water to clean it. That is about the only thing you need to remember. Putting other types of ink like Indian ink, Chinese ink, powdered ink, or any other types of inks or liquids will ruin your pen.
Fountain pen ink is made of mostly of water along with the color pigments for the various colors. You can pretty much buy inks anywhere from local pen shops, craft stores, office supply store, or online. These stores will carry plenty of brands and colors. Many in the fountain pen forums prefer Noodler’s Bulletproof Eel on the less expensive side and the higher price Pilot Iroshizuku Kon-Peki . But there are other brands from Lamy, Waterman, Montblanc, Parker, and others to choose from depending on your preference in color, shading, saturation, time to dry, and budget. You don’t necessarily have to pair up the pen manufacturer with its ink, i.e. Lamy ink in a Lamy pen.
When changing different ink types in your pen or after refilling 2 to 3 times, you should clean it out first. See this post on how to do this.
You should properly store bottle ink in a cool and dark place. Don’t open too many bottles up as you’ll want to use the open bottles as soon as possible.
Experiment and try the different inks and color. Find the combinations of pen and the best fountain pen ink that makes your writing experience enjoyable.