I was hoping for some additional information before answering but don't want you to go without an answer either. Usually this kind of behavior is due to dominance aggression. If you spay the younger dog, you should resolve the issue pretty quickly since the younger dog will not be intact and thus not a good choice to be the top dog. This will keep the present top dog in the alpha position.
The other method is obedience training each dog so you can control their behavior and stop fights before they start using commands and behavior modification. You will also need to learn body language in order to give commands before the fight starts. Here are some sites on that.
The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below.
Additional training can also help. Both dogs should be leashed and if one dog even looks at the other dog, a correction should be done. Any sign of aggression including a prolonged look, hair raised on the shoulders, a growl or even a stiff legged walk, should be corrected. A correction is a quick tug of the leash and a firm low toned "NO". Once you have done this couple of times, you should notice the dogs ignoring each other. When that happens, you will want to reward them for the desired behavior. Again, use tasty treats like the hot dog slices. This teaches the dogs that you WILL not tolerate fighting in YOUR pack.
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Since there have been recalls on certain foods, please check the following site to be sure the food your animals eat is not affected. If it is affected, contact your vet as soon as possible. Have your dog seen if they have any symptoms.