When I woke up this morning I found a dead Long skirt black tetra. I was puzzled as to why he was dead....this afternoon I noticed my biggest White skirt tetra was nipping at and ramming one of my long skirt black tetra and then I started wondering if that was the cause of death for the other long skirt black tetra I found this morning. Is this possible and why would a fish all of a sudden get aggressive like that? Any idea? Thanks.