Fountain pen ink by the bottle, can be used in any fountain pen that either has its own filling system, like piston- or vacuum-filled pens, or in most cartridge pens by fitting a converter. It's usually much more cost-effective than using cartridges, avoids the plastic waste, and is available in way more colours and ink types, opening up options like sheen and shimmering inks and waterproof inks, if you're ok with taking a bit of extra care.
