very fast fin rot

Today the sick female betta died. Tail rotten all the way. Another betta is also sick with the same disease. Wonder what kind of fin rot is this? So fast within 12 hrs kill the fish...

fierce puffer

Bought 2 plants, 2 more ottos and 4 female bettas (on the cheap) from Vista Point fish shop. Placed them in the same planted tank with my puffer and my fierce puffer starts to bite all of them! Although the bettas bite back sometimes, they are no match for the tiny puffer. I had to isolate the puffer in a breeding cage for baby fish. One of the female betta's developed a nasty fin rot. I wonder was that due to the injury inflicted by the puffer?