I have three fish. One (the smallest) is always peaceful, one is so aggressive I had to separate her from the tank, and another who is only aggressive around feeding time.

First, what would cause my crazy-aggressive fish to be so aggressive? It's unprovoked, and it's like watching an animal with rabies. When she was with them, she'd go after them in earnest, nipping ferociously at their fins. She wasn't always like this; when I first got her, she got along with the others fine. Brain damage, perhaps? Or something else?

Second, what could I do to keep the smallest fish safe around feeding time? I had originally been worried about the size difference of my two tank fish, so I'd been keeping an eye on them. When I feed them, the smallest fish doesn't get enough food, so I've been separating them before I feed them, taking the bigger fish out while the smaller one eats.

However, it takes a few minutes after I put him back in the tank before he stops bullying my smaller fish.

Help?