Black Moore in a 30 gallon?

[Flaringshutter]

I think you've had an unusual experience with singletails and koi. Koi get along fine with comets, watonai, and even ryukin - they have no problem with longfin goldfish, and I've never heard of them eating the fins. I think what you're seeing is finrot from LFS keeping them in poor water conditions. That's quite common.

 

[RodInCALIFORNIA]

i have been keeping goldfish for 20+ years and no its not fin rot i have watched the koi swim up and just attack the long fins finally the LFS has stopped housing the koi with the long finned goldfish.

 

[crimsonmoon]

Nah, goldfish in general are very sociable and happy-go-lucky guys. The ryukin would be fine, he'd just make new friends.

perfect! another great reason to love goldfish haha