My black skirt tetra fins are torn by other fish why?

I had 2 black skirt tetras and had them for 1 year with 2 gouramis, 1 zebra danio, and 1 tiger barb. But when I moved them into a 20 gallon from a 10 gallon they started to bully the black skirt tetras. They were fighting so bad that they broke a fin off and then ate 1 of the tetras alive. But since then the bullying has stopped a little but it continues. Why? I have plenty of hidins for them great water quality live plants too.