Wildlife pond and mysterious fish.

Hi Phoebe, I just came across your question which is a slight mystery to you. I guess there need to be a few questions you need to answer and eliminate first  1) Is you pond fed by a small stream, 2) have you bought in any pond weed 3)Where is the nearest stream another pond or ditches to your pond. 4)Do you get ducks or have you seen a heron in or near your pond? 5)Did you fill the pond from another source other than tap water? 6)Did you have flood water in your pond during last winter's floods? All the above have potential to bring in fish eggs or small fish fry. Did you take a photo of the fish or do you know what species it was?