Thank you for the additional information. It is helpful.
If the one dog was just fixed in June, you should start seeing a reduction in the aggression level. However, let me explain what is going on with them. Dogs are pack animals and there can only be one male leader and one female leader and among those 2 dogs one is the ultimate boss. When you have 2 females the older dog is usually the alpha especially if they are not spayed. However, as an older dog matures and becomes a senior, they become weaker and often have medical issues. When this happens a younger stronger or even larger dog will feel like they are a better choice to be the leader. When that happens they will disrespect the boss dog. They will do things that they didn't used to do like push past the lead dog to get attention or even get too close to the leader. When that happens, if the leader fails to reprimand them, they end up becoming the boss. Most alpha dogs do not want to give up their position.
In this case, you have a stronger younger dog who is likely becoming the alpha since the other dog is older and smaller. She doesn't want to give up the top position and even if she does the younger dog will take every opportunity to show her that she is the new boss and thus you will have some fights for no real reason at all.
What you need to do is feed the alpha dog first, show them attention first and even let them in and out first. Start obedience training both dogs all over again. It doesn't have to be a class just formal training. 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.
It takes about a month for the hormones to normalize after spaying, so the younger dog may naturally become more submissive when that happens. I'd do the training anyway as it is always a plus.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer positively so I am compensated for my time.