I think you have no choice. He is her father and better to let her know now then to ask you someday and find out you just didn't want to tell her or something. It doesn't mean you have to let him be in it enough to influence her or be a bad example to her. Just let him visit her and be her dad...
angel fish would be my first suspects and then the tetras
angel fish are actually pretty aggressive and poor Betta's are only aggressive against their own kind.
take him out!
