First, fish are a complex animal that has evolved over hundreds of millions of years to take advantage of the food sources that have naturally been in their environment. Plastics have been in the environment only several decades, nowhere near long enough for such a major change in fish digestion.
Bacteria, on the other hand, have been found that are able to break down PET (the kind of plastic in disposable water bottles). But bacteria are *much* simpler, and are able to evolve more quickly to meet the demands of their environment.
Second, plastic is unlikely to be a sufficiently nutritious food source for animals with the energy demands of fish.
Now, it might be possible that *if* PET-eating bacteria become better at breaking down plastic and/or breaking down other plastics, and *if* the byproducts of their breaking down of plastic is biochemically useful to fish, and *if* they are able to colonize the guts of a fish without being themselves destroyed or killing the fish, then *maybe* a species of fish might develop the ability to digest plastic. However, that's a lot of ifs, and even *if* those aren't an impediment, it's not going to happen any time soon.
It's called evolution, not revolution. Changes are slow, incremental, and build up over time, not sudden and explosive. If an environment changes too fast, animals go extinct. They don't adapt instantaneously.