I've been using the Petzlife gel for over a year now, and it seems to be doing a great job keeping the tartar off the teeth. You use very little, and don't brush with it like you would with toothpaste. The Petzlife is said to soften tartar, and you then brush it away at a later time. I also use the Merrick Bully sticks because they a very hard, and I think they help scrape the tartar off teeth. I would be extremely wary of any product that you could just add to the water to clean teeth. Why wouldn't there be a human equivalent? Here's a link to the Petzlife website, but you can buy it elsewhere for less. When things sound too good to be true, they usually are.
PetzLife Products Inc - Products For a Healthier Pet