Well many things can cause bad breath. I am not sure if Royal Canin has fish oil in it but this can also cause a fishy breath. Bad teeth can cause stinky breath also. My Yoshi had stinky breath forever and I would take him to the vet and have his teeth cleaned or pulled, It frustrated me so bad as the kisses were really smelly, LOL! Finally I took him to the vet and talked to him and we discussed some of the reasons the breath could smell and he asked if his anal glands needed expressed. I told the vet that I had never smelled Yoshis glands and he has never scooted or released them anywhere. The vet expressed them anyway just in case and he got the nastiest pasty mess out, he said that it had been building up for some time
Apparently Yoshi would clean that area at night when I was asleep so it gave him that disgusting odor. Once the vet expressed his glands the odor was gone for months
When ever his breath starts smelling again I just express his anal glands and no more odor
I just thought I would throw that in as you never know, LOL!