You can let them go at it for now, if you see that the aggressive girl is just rentlentlessly going after one girl then put her in time out. If you've got a breeder box or even petco/petsmart cups left over put them in those and float them in the tank. The most important part is to float in the tank so they can see each other but do no harm. Then eventually the aggressive one will see that she can't get at the girl and then forget about it, it might take a few days up to a week or two. If you put her in a cup just change her water every other day, you can just dump it into the tank (since she's not being QT'd) and scoop her out again and cover the cup to let it float.