In general you can use any brand of ink with any fountain pen, although some manufacturers state you get the best results when their ink is used with their pen. If you are using bottled ink in your fountain pen you will need a converter which is basically just a refillable cartridge that fits inside your pen. The choice between using bottled ink and ink cartridges is entirely personal preference but there is usually a much wider colour choice available with bottled ink (although slightly more fiddly to refill your pen this way). Diamine, in particular, have an amazing range of colours.
Rotring 23ml high quality Black ink Bottle, for use with Rotring isograph pens also available in 250ml£5.95Rotring 23ml high quality Blue ink Bottle, for use with Rotring isograph pens£5.95Rotring 23ml high quality Red ink Bottle, for use with Rotring isograph pens£5.95