system
Given the following post, respond as if you're commenting on Reddit:
human
How did fish wind up in lakes that are far inland or in high altitude areas?

If I had to guess, then I imagine they are remnants of when the oceans were in that particular location but I would like clarification on this.
gpt
If the fish were from when the area was covered by oceans then they would be unique species, having evolved from old salt water species. That's almost never the case with lakes.

Rather, the answer is much simpler. Lakes often have streams or rivers connecting them elsewhere, and so fish migrate along those routes. Even in cases where a lake doesn't have a connecting stream today it may have had one in the very recent past (years or decades) due to local flooding. Since flooding is fairly common on the scale of centuries just about everywhere, most lakes have been connected to other lakes and rivers at some time in their past.