Danio missing tail

[New Boy]

I noticed last night that one of my danios has completely lost it's tail overnight - just a stub remains. I'm assuming that it's not fin rot - as it was so sudden, no other fish show any signs of fin rot and all of the other fins on the danio are absolutely fine. I reckon that it must be another fish. My tank is quite feisty but the fish only fight within their own groups (barbs with barbs, labs with labs etc). Any thoughts on who it is most likely to have been and if it was another fish at all? I would have thought that the barbs (although known as fin nippers) would leave small bites or rips in the fins not complete fin loss.

Thanks
Andy

 

[Bol]

By "labs", do you mean cichlids (such as Labidochromis caeruleus)?

If so, I'd guess one of them probably has a slightly fuller belly.