Can eating fiber stop ketosis?

The short answer is no: fiber itself won't stop ketosis. Even though fiber is considered a carbohydrate, it is not broken down by the body into simple sugars, does not trigger an insulin response, and does not spike glucose levels. 

However, keep in mind that many fiber-rich foods may contain a lot of natural sugars. So, on a low-carb diet, you can get fiber from foods like leafy greens, nuts, seeds, cabbage, broccoli, cauliflower, and avocado.