My rats have started fighting?

I have two rats, Dexter (White dumbo) and Tony (Russian blue) They are 6 months old. Since I have had them they have fought each other. Up until recently it has just been play fighting where they pin each other down and groom. The other week I found a scab on the back of Tony's neck, then over the next few days there were more, I kept a close eye, Tony always seems more dominant but they both get on, they follow each other round and snuggle together. Then I noticed a scab on Dexter's ear, this is now a small tear. The definitely are fighting although I haven't witnessed it despite spending hours watching them. They both have little scabs. I don't want to split them up because I know they do get on and have heard that it could be worse for them being separated from their companions. I cant afford to get another cage and don't have the space. Dexter also has respiratory problems which is a priority at the moment, so due to this I can't afford to get them neutered unless really necessary because of his on going treatment at the vet (Very expensive). I was just wondering if this was just an age thing that they will grow out of. Other than all this they seem very happy and possibly the most loved rats in the world