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Hi Vanessa, I'd like to try and help you with this issue. It can be very stressful for everyone involved when inter-cat aggression happens. So the first thing I would suggest would be to get a couple of Feliway diffusers and plug them in the areas where the cats spend the most time. I would also make sure you have separate litter boxes/food and water stations set up in different areas of the house so you can minimize stress and aggression associated with resource guarding. Sometimes you have to go back and start over "from scratch" as they say. Remove the other cat to a separate room for at least a week or two so that everyone has a little time to calm down. Next, rather than turning them loose together, I would suggest either mounting a screen door on the door to the room that the new cat stays in (I know, this is a big deal) or buy two child gates and place them on top of each other in the doorway. The idea is for the cats to have limited access to each other but be able to smell the presence of the other cats and (hopefully) get used to the presence of the other cat. If the mere sight of the new cat leads to aggression then you may want to start with a piece of cardboard covering the bottom 2-3 feet of the screen door or child gate. (Again, the idea is that this is a little more permeable than a solid door and over time, you can cut away the cardboard.) In addition, you may want to feed both cats something very special, just a spoonfull or two, every day on either side of the door so that they associate something very pleasant with the presence of the new cat. This could continue for weeks depending on the response of the cats. You can also, periodically remove the new cat from its room and confine the older cat to that room so that he can familiarize himself with the smell of the new cat. (This only needs to be done for a few minutes a day or so, if you don't like confining your older cat. You can also exchange bedding, toys, etc between the rooms so that all cats can explore things that smell like the other cats. If they will tolerate it, another idea is to take a towel and gently rub each cat with the same towel periodically, exchanging their scents, so that each cat smells a little bit like the other cats. There are also anti-anxiety medication options, sometimes it is better to medicate the victim cat,especially if it shows a lot of fearful avoidance behavior. Sometimes that fearful behavior is what triggers increasing aggression in the other cat. However, I am not sure that you are seeing that, based on your descriptions. What it does sound like is happening is just normal behavior from the new cat. Most cats are not comfortable with the sudden appearance of new cats on their territory/space and it can take a very long time for them to become comfortable. (I have seen it take months and once even a year for new cats to relax around each other) Sometimes, some cats will never learn to live with other cats, unfortunately you don't know until you start trying these various methods.
Good luck and let me know if you have any further questions.