Fish infection/disease? PLEASE HELP!!!!! Wat is it, and wat is the most humain thing to do?!?!?!?!?!?

Ok first i got these two new fish, and for like two weeks every one was fine. Then this one fish started nipping the others, and he started to get one of my original fish to start doing it too. The first nipper died. Then, the second nipper nipped two of my other fish until they had red spotts all over, and at the bases of their fins. And one of the nipping victoms started getting this white stuff hanging off his tail fin, like he was loosing the "membrane" off of his fins. Then i got a divider to stop the nipping, but the nipper just jumped over it, so i took it out. And now my fish are acting all strange. Like they will lay very still at the bottom of the tank, and when i lift up the lid to feed them only one has enough strenght (another trys but just cant) to come up to the top to feed, the others just wiat for the food to sink, and lastly today when i came home they ALL had that white stuff comming off of them. I think they are dying. PLEASE HELP!!!!!