You dig a big enough hole that water can stay in, it becomes a pond. And once it's been a pond for long enough, fish somehow appear. Even though it's not connected to any other water - hell, consider all the lakes that aren't connected to any other water. How the fuck are there fish? Where do they come from?
One day, something in space is going to look at humans the same way. You go to a new galaxy and there's freaking humans in there, like they just spontaneously manifest on random planets that have the ability to sustain them. All you need is an atmosphere with enough oxygen, some form of water, and that's pretty much all you need before they seem to just pop out of the ground and start terraforming it.
The mystery of the lake fish has been solved, by the way. It's waterfowl. Much like the birds that eat seeds and spread them around, waterfowl consume roe from the water, and while the number of fertilised fish eggs that pass through the digestive system of a duck or wild goose alive and unharmed may be small, it's not zero. A goose will shit in the lake water, and through comes the roe. It happens just often enough that lakes and ponds become - and remain - populated with fish.
Humans don't pop up on unknown planets and and unreachable galaxies on purpose. They couldn't get in there any more than they could get out of there, they have no choice but to claim the most viable-looking planet they can reach and start terraforming it. But how did they even get there?
You see, every once in a while an unfortunate human spaceship gets swallowed by a cosmic duck
Some fish are also just able to straight up walk on land
"They just walked there" is now my favourite theory of "how are lakes populated with fish".
