That happens to me too! It has happened multiple times. Yuuki, my Halfmoon is too sweet and docile to attack, but my other Crowntail has shredded his tail. But no one has been seriously hurt. And every time I fund Yuuki on Ryu's side he just comes up to say hello.
*2 Bettas (for now RIP Ryu) *2 Goldfish *9 Guppies *1 Gourami *2 Chinchillas *2 Budgies *2 Dwarf Frogs *1 Dog *1 Cat *2 Hermit Crabs *2 3gs *1 10g *1 Pond *2 2gs