My molly is nipping at one of the cherry barbs in my tank. What should I do? The barb is losing colour

Separate them immediately! If a fish is losing pigment then it is getting damaged terribly. My grandparents are big fish people, they also said this is bad. The fish may require more food, so first move it to a different tank, but feed it more as a last resort.

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.