Ink additives are, as the name suggests, substances that you add to fountain pen ink to change its properties. There are three main categories of additive; some hobbyists may refer to these additives by brand-specific product names, but any product from the same category should be just as effective.
These are surfactants which act to make an ink flow more readily by reducing the ink’s surface tension. They should be used in tiny quantities.
The recommended approach is to add a tiny amount (a drop on the end of a toothpick will do!) to a converter or cartridge of ink, or a couple of drops into a sample vial if you’re feeling brave. Don’t add wetting agent directly to a bottle of ink just in case you slip or make an error.
Example products:
Alternatives:
Some people recommend a drop of dish soap as a cheaper alternative, as it's a surfactant and will have the same effect.
However, recent discussions suggest it may encourage mold growth.
These dilute the colour (i.e. reduce colour saturation) of an ink without altering its other properties such as lubrication, wet/dryness, etc. These are used in much greater quantities than wetting agents – Van Dieman’s recommends no more than 30% diluter, but you can add much more than that with no issues if it results in the colour you want.
The recommended approach is to add an eyedropper-full to a sample vial, but you can experiment using a plastic artist’s palette, using each well to add different ratios of ink & diluter and then swatching the result.
These products are also ideal for rehydrating an ink that has evaporated or dried out.
Example products: