Fleas....
Unless my dogs kill a rabbit in the yard and eat it, I've never had a flea problem on any of my pets. Part of flea control comes from the environment the dog is in. Manicured lawns and flower beds don't tend to harbor squirrels and rabbits, major carriers of fleas. Dogs who go out in the tall grass, fields or woods are always more at risk for parasites. Perhaps this is why I've never had an issue. I've had dogs for twenty one years now and I'm a professional groomer and I've seen a lot of fleas on dogs in my home shop and my dogs still haven't been bothered.
I would stay away from any Hartz product. Cheap, cheap, cheap and I've heard horror stories about pets dying in the past when certian products were used to control fleas/ticks. Since every dog is an individual garlic may or may not work for every dog. It depends on how they metabolis it in their systems, fleas are attracted to carbon dioxide when your pet exhales and the heat of the body. I know this from first hand experience. Watching my dog kill a rabbit and the minute the rabbit ceased breathing those little buggers marched or jump straight up my dogs muzzle.
Good luck.