I use "Dental Rinse" from Petsmart, it works wonders on mine, 8 yr. 1 1/2 yr and 6 yr. with soon to be 2 more. You mix it in thier drinking water, they can't taste it as far as I know, or smell it. I started using it @ a year ago and they never seemed to notice. It took about a week and all the stinky breath was gone, and my older ones had severe "fish breath", it would knock you over!
I mix 4 tablespoons to a gallon of water and keep it in the frig.