I've not tried this myself, but ProDen PlaqueOff is made from seaweed and looks promising: ProDen PlaqueOff™ - For Healthier Teeth and Gums

I know they sell it at Chemist Direct, think they're the cheapest. I'll get some for Wilson, because even though he eats only raw with bones I've noticed his teeth not being as shiny white as they used to be - and prevention is always better than cure

Be aware it's not suitable for cats with thyroid problems as it contains iodine.

They've also got a version for humans

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