I had a few minnows and goldfish in a stocktank.
When I upgraded to a larger stocktank I used the
occasion to separate the two species. Problem is,
two of the "minnows" kept growing and started
to change color to something suspiciously like ...
goldfish.

Eventually I moved the babies back into the goldfish
pond and one is almost a carbon copy of the white
goldfish with red markings; the other baby is a
carbon copy of the solid gold goldfish.

No casual reproducing has gone on this year, or it
has and the adults got the babies. But it was fun
watching the baby goldfish grow and slowly change
colors to match the adults.

Gail
near San Antonio TX