I apologise in advance for the length of this, but I think I need to give a bit of history first!

We got 2 rats a few years ago, one died shortly after, so we ended up getting two sisters from a rescue. The original rat (Blondie) was about 6 months when we got the sisters, who were about 3 months. We introduced them immediately, and they were soon inseperable and there was no fighting. When the sisters were about 2 years old they died (severe chest infections despite many months of treatment) and we went to get some more rats to keep Blondie company, and found 4 sisters, about 12 weeks old. Introduced them and they all got on fine (Blondie was always the dominant one) and lived happily together for about a year and a half. We had to have Blondie put down a few months ago (she was about 4, had a huge mammary tumour and severe arthritis in her back legs, yet still eating like a trouper). The girls coped fine, and one of the sisters became the dominant one, with no problems. Unfortunately, they were back and forth to the vets (they all seem to be getting mammary tumours) and at one visit, the dominant one coughed up a load of blood and then died at the vets. Was very sudden, and we have no idea what caused it. Now to the problem - the rats are now fighting a lot. One of them is covered in cuts - a few on the back, but loads of the face and neck. None of the others are cut. We think we know which one she is fighting with (one is a hermaphrodite and the bottom of the ladder). What can we do to stop this? They have lived happily together for years, so what can we do now, to stop the fighting?