Is Octopus Ink Similar to Fountain Pen Ink?
"Sepia Ink
There is one ink that is related to cephalopods: the historical artists' ink sepia, one of the brown inks used by artists for their pen and ink drawings. Sepia is a red-brown ink made from the ink sacs of cuttlefish, which are dried and ground to a fine powder, then mixed with shellac. (The ink takes is name from the cuttlefish species Sepia officinalis.) This ink came into use in the eighteenth century and was quite popular in the nineteenth century. It is sometimes difficult to identify a true sepia ink drawing, since other brown inks were in use as well. True sepia ink is still available from specialized artists' supply houses on the Internet. The term sepia now also applies to any red-brown color similar to the color of sepia ink."
http://www.tonmo.com/articles/octopusink.php

The fountain pen friendly version of Sepia (Squid Ink) is available from Hakase - there is some more information here in the Japanese Inks thread.

Other types of squid ink are best used with dip pens etc.