Question
QUESTION: I have a pond that I inherited upon purchase of my home, it has three goldfish..It is a manmade pond purchased probably from home depot or a store like it..It has a filter in the middle kinda like a box with a plastic tube on top and water comes out of it.we have cleaned it a few times..we have green algae around the plastic pond and were told it does not hurt the goldfish.we went away for a few days and when we came back we found we now have these little black fish all around the pond eating the algae, they also eat the fish food we put in but not right away..are these fish okay and where did they come from we did not purchase them.is there something we should do to get rid of them..we also have frogs around the pond which just appeared.no problem but quite puzzled about these little black fish not bigger than a figernail right now..are they okay in the pond..anything you can tell me is helpful...thank you Lisa

ANSWER: It sounds like you may have Gambusia.  They are awesome to have, they eat mosquitoes, and they do no harm in a pond.

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QUESTION: thanks so much..wonder if you can you tell me where they came from..it is really interesting and why or what has caused them to be in our pond...pretty cool...thanks again for anything else you can tell me...Lisa

ANSWER: The most likely explanation is that they came with a plant you purchased.

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QUESTION: I imagine you can not answer but I just wanted to let you know we purchased No plants for our pond..I hope you are correct and they are Gambusia..Though again I can not imagine where or how they came..The only thing we put in the pond is the fish pellets..That is it..We put nothing else in the pond..If you think of another reason why we got them, please let me know..In any case, Thank you so much for your assistance.Lisa

Answer
A few more far fetched guesses...

In a storm during a high water level, the fish could have come from a nearby stream.

Gambusia are live bearing fish, but if it is something else (that lays eggs) it could have very easily come from a bird passing over the pond.