@23moostveen I found this site that talks about certain goldfish, not koi that do not have dorsal fins.
Fin Variations
There are really only two major caudal fin variations, single and twin.
Within these two types, caudal fins can be long, short, pointed or rounded at the ends, forked or have no fork at all, depending on the goldfish type. The rest of the finnage generally follows the caudal fin form.
Single tailed varieties include the
Common,
Comet,
Bristol and London shubunkins.
All other fancy varieties should have two caudal fins (and two anal fins on the better specimens).
Sometimes single tailed fancy goldfish are called nymphs, but they are only poor specimens that shouldn’t have made it passed the first
cull.
The only other major fin variation is goldfish with a missing dorsal fin. These varieties include the
Celestial,
Water Bubble Eye,
Pompom and
Lionhead. These goldfish are some of the most highly developed.
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They breed these fish to not have dorsal fins and cull out any that do. I would not like doing that. I think I would just enjoy your fish as it is cuz chances are you would not find another one to breed it with. Also I read somewhere that the dorsal fin helps the fish to stay upright and not have a tendency to roll over.
