Ink India ink is used because it produces a dense black line. Some india inks, especially the cheaper kind, can be thick. This will cause problems when the ink starts to clog your pen or brush. As india ink ages, it thickens due to evaporation. A drop of water will fix this, but use distilled water.
Tap water causes the particles suspended in india ink to clump.
A simple quality test when buying ink is to draw a line with a brush or pen and wait until the ink dries. Hold the paper up to the light. If the dried line appears to be slightly raised from the paper then it has lots of shellac, which is an important ingredient in ink. If it looks like it has soaked into the paper, it does not have much shellac and is not as high quality. Erasing ink can be difficult.