Here's a great link the explains about registries.
Registries
This is what it says about the ACA
ACA - American Canine Association Advertises that over 3000 adult breeding canines are registered each week. Only registry endorsed by Petland. **Caters to commercial breeders.** Refers puppy seekers to pet stores.
You can be sure that the dog that is ACA registered has parents/grandparents living in puppy mills. I think they finally closed Petland down, but it opened under a new name. Because people don't understand the differences between a reputable registry and a paper registry, they think that any dog that is registered is purebred. This is not true, as many paper registries will even register mixes. Anyone can open a registry. The only reputable registry in the United States is the American Kennel Club known as the AKC, not every breeder who registers with them is reputable, but it's a starting place. The only way you can ensure your dog isn't coming from a puppy mill is to see for yourself where it's been raised.