Hi
You most likely have the situation worked out. Redirected aggression is quite common in cats. You’ve already tried the simplest fixes with the pheromone plug-in and comfort zone products. Unfortunately this form of aggression is very hard to fix. I’m not going to be able to give you an easy single solution for this without using medication (I’ll discuss that later) but I’ll go through some basic techniques.
Removing the ability to see or sense the outside cat (and deterring it) and fulfilling the social needs of your own cat are the important parts to solving this problem.
Cover any windows, remove vantage points that allow her to see outside, restrict her to indoors or whatever other measures might prevent her from seeing/contacting the outside cat are all important steps.
Environmental enrichment is very important. It works to reduce anxiety in the cat but also acts as distraction therapy. So this means lots of toys, playthings, climbing apparatus and quality time with you.
Sometimes putting a bell on the cat with the aggression can help by allowing the “victims” the ability to hear her coming and avoid her.
Try alternating their bedding, food bowls, etc so that they can become used to each other’s smell.
Total separation rarely works but sometimes a “partial” separation perhaps using a glass door or even a crate can allow for visual or smell contact without the risk of a fight, allowing time for things to settle down. Similarly training them to be controlled by a harness and lead can allow for some controlled contact. Feeding treats whilst they are allowed this controlled contact as a distraction.
Most importantly never use punishment.
If you find you are getting nowhere you will need to consider medication and that means a vet visit of course. Don’t be put off doing this for fear of “drugging” your cats. The alternative is to have them leading a life of fear and anxiety. I would suggest fluoxetine (Prozac) for the aggressor and perhaps clomipramine (Clomicalm) for the poor victims or even Clomicalm for all of them. Clomicalm is an anti-anxiety medication tailor made for anxiety issues in cats. You would need to see your vet to obtain it. If you are in the USA it is not registered for cats (just dogs) but this is a licensing issue. It is used all around the world in cats and most US vets will prescribe it for cats. It must be used for several weeks to have its effect so I generally use it in combination with Feliway. So no quick and easy cures here but I hope I’ve helped in some way.
Kindest regards, Peter
Cat Veterinarian
Bachelor of Veterinary Science (University of Melbourne, Australia)