There are several cases in which fish have inhabited new bodies of water without their origin being evident.
One example is Thailand, where a shopping center was built without the proper permits and therefore was never allowed to open; over the years, the underground parking lots and the ground floor have been flooded with rainwater and are now full of fish.
The usual suspects are birds and bats that carry fish eggs, but fish are also known to have the ability to travel long distances in muddy conditions during torrential rains.
In Australia, there is a species of fish that can live for months underground, only in damp soil, and then return to the surface to swim in floodwaters, burying itself again when the waters dry up.