Virbac Animal Health C.E.T. AquaDent Drinking Water Additive is the BEST I have found - I tried the ones from Petco & Petsmart - they don't even come close to this Product!
My Vet suggested it to us & he sells it there at his office!
It reads to add 2 teaspoons to a quart.
i add about 1/2 teaspoon to their lil bowl of water.
http://www.cetdental.com/