This has never happened before. My fish are in a 20 gallon (76 liters) 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.