Fish... sticks...

I don't know what my problem is and I hope this doesn't reflect on my ability to care for people in the future.
MY FISH KEEP EATING EACH OTHER!
This happened to me in September. Gabi kept chewing on Gus's fins. Gus eventually died and Gabi fed off his carcus for the next couple of weeks because I was mad that she ate him. Somehow Gabi lived way past my expectations. Well I bought Lumierre about a month ago and he and Gabi lived together for 5 days. Gabi was floating at the tank. I don't think she could handle being with some fish after being alone for so long. So the next day I went and bought Gypsy. Well Gypsy had some nibble marks but nothing major until this weekend. I'm pretty sure she is going to die soon. Her back fin is completely gone and she's having trouble swimming.
I feed my fish 2 times a day. I leave the light on. I use the water purifier. Why does domestic violence continue to occur?
If I can't keep my fish alive, how am I suppose to protect my family in the future? They'll just end up eating each other and then be hauled off to jail. Man, my future sounds bright.