There’s minerals and other things in tap water. These things will cause issues and damage.
Though, the Lamy Safari is a cheap enough pen to not worry about buying a new one after causing damage.
However, there are other considerations for not using printer ink. Printer ink is designed to be used on specific paper, while fountain pen ink is designed to be more versatile.
The colors for fountain pen ink are also more than the base color. There’s the shading which can be a different color from the base The sheen, which can make the ink shine a different color in different lighting. It’s designed to flow properly through the feed with an appropriate level of viscosity. There are a lot of factors and testing that goes into fountain pen ink. You can read more about it here: www.jetpens.com/blog/…/968