Have had aquarium for a very long time but new to having Piranhas. I initially bought 3 about the size of a quarter. I have a 65 gallon tank with loads of hiding and all real plants. They were the only 3 they had. 2 weeks later I bought 2 more. They were about the same size. Well long story short. The 2 I bought last ate the other 3 even though I have around 20 feeder fish in there with them. The 2 that are remaining, one it missing his entire back fin even though he is the bigger of the 2 (only slightly). He seems to be doing quite well and still is aggressive and swims ok. Will is tail fin grow back if he survives?