Microparticles of plastic in fish, in most cases, come from the water exiting a water treatment plant that is entering the environment. Some of these microparticles are manufactured, often in beauty or cleanser products, others are the result of plastics breaking down.

While tap water may contain these microparticles of plastic, there's no reason to believe that water out in the wild will be free of them.

https://en.wikipedia.org/wiki/Microplastics