Are fish ponds contagious?

[Hotzcatz]

I expected tiny water snails to mysteriously appear out of "thin water" but there now seems to be fish that have spontaneously appeared in my stock tank pond. [:0] I had first thought my neighbor had come by and added them in as a mosquito deterrent but she says she didn't sneak in fish. As far as we can figure, when she brought the plants over in a plastic bag (just damp plants, no real water) there must have been fish eggs on the plant roots? Is that possible? She brought me plants from her pond with comet goldfish in it and the pond with swordtail fish in it. There are now a dozen small greenish fish with split tails kinda like comets but green instead of gold.

There are also some "rams horn" small snails in there now. I had expected those to just spontaneously appear but these fish have me puzzled. Anyone else just have fish spontaneously appear?