It' just bruising on their mouth. Rainbowfish have soft mouths and regularly bump into things when moved into a new tank, especially if the tank is smaller than their previous home. Clean glass also encourages this because the fish can't see it and swim into the glass, bruising their lips.
There is no need to use chemicals for this and regular water changes and gravel cleaning will normally prevent it becoming infected. The lips should heal up over the next month.