This has never happened before. My fish are in a 20 gallon tank, they've been fed the exact amount every day, and none of them have had problems before (other than another incident with my (now moved) pearl gourami). This came relatively out of nowhere and I'm decently new to owning fish. Should I buy another tank? What can I do to keep the molly from acting out?
Look for a fish breeding box as a temporary solution. You can get a small plastic container with holes in that will sit in the tank and give one fish his own private space without requiring a second tank set up. Personally, I'd isolate the trouble maker and give the injured one time and comfort to relax and establish territory (if they do that).
I used to use a cheap breeding box to give my fertile female goldfish a break. I wasn't breeding them, but they used to get pushed around a lot when they were fertile, for about a day every two weeks for a few months in summer.