The physical differences between goldfish and koi carp
While goldfish and koi are very different and quite distantly related, it can be difficult for the uninitiated to tell the difference between the two. However, it is not difficult to tell koi from goldfish once you know what you’re looking for.
The main differences between the two types of fish are:
Goldfish are generally smaller than koi when adult.
Goldfish can be seen in a much greater variety of body, fin and tail shapes than koi.
Koi tend to have a greater range of color and pattern combinations than goldfish.
Koi have clearly visible barbels near their mouths, which are missing from goldfish.
Goldfish have a split caudal fin, while on koi, the caudal fin is joined.
The dorsal fin of the goldfish is detached and free at the back, while on the koi, it is attached along the whole of its length.
Koi are generally much longer lived than goldfish and while the two can interbreed, the resulting offspring are always sterile.