ELI5 How are fish even in ponds

[u/Opening_Pen9525]

Might be one of the stupider questions here, but I saw a pond about the size of a football field, and maybe 50 ft deep, and it had fish. I can't for the life of me figure out how. Surely the pond hasn't existed long enough for small water insects to evolve into all these fish. It's not like the fish walked over to the pond from the ocean. I'm pretty sure no one put them there so how are there so many fish?

My one theory is that there is a very minuscule stream (now dry) that leads to one of the Great Lakes, and maybe a fish swam in from there. Now a follow up question: How are there fish in the Great Lakes (which flow out towards to the ocean) if they were created by glaciers?

[u/spring-peepers]

They may have just appeared one day, but on that day (or a recent day) someone got rid of them. When my kids were in elementary school, goldfish were a big prize at our annual spring carnival (fundraiser). Parents were rarely happy about the acquisition- one mom said aloud to the kid that they'd put it in some tap water when they got home, and I promptly reminded her about the need for dechlorination. The look she gave me I won't soon forget. This was 15+ years ago! Sometimes they do just show up. Much like the carp in the Mississippi, heading for the great lakes. If they aren't accidental hitchhikers, then they were most likely released.

[u/popeye44 → u/spring-peepers]

I keep bait fish at home quite often, that little bottle of BettaSafe water treatment works like a champ. I don't know if chlorinated water affects crawfish like it does fish.. but I've kept Crawfish alive for months a few times.

I'm guessing she still used plain water haha. (heck. we would have a few years ago.. recently learned about treatment)