Please I need help. My dog has become fixated on my other dog and then attacks for no reason.?

You have a breed that was bred for years to be dog aggressive. Unless this is carefully, selectively bred out, and it is not being done nearly often enough by the Pit Bull breeders, you have a dog who is by nature aggressive. Females often decide to be the top dog and can be fairly aggressive about it regardless of breed. I suggest you find an experienced behaviorist/trainer to work with the dogs and see if it can be corrected. If not, please consider rehoming one of the dogs.