Which fish nipped my goldfish?

i had: 2 goldfish, 3 zebra danios, 5 neon tetras, 2 black widow tetra's and 2 platies. the one day i found one of my goldfish had its tail torn(tail was quite long) and a few days after died. about 2 weeks later my smaller goldie died too from the same thing. anyone know which fish would have done it so that i know not to buy them again?? thanks