Hey guys, first time posting in this sub reddit so any help would be amazing. So me and my partner have a cat that is roughly 3-4 years old that is super sweet relaxed cat and extremely friendly. Recently my brother was taking care a colony of feral cats at his place and one of the cats took a real liking to him, was a small 6-8 month kitten/cat and he was really friendly or at least seemed so. We decided to take him, he was originally pretty docile and super cuddly, would climb up on us and sleep and such.

We kept him separate and slowly introduced him, everything seemed to go well although our older cat, Pazu, was pretty angry in the beginning, he stopped worrying much about it pretty quickly. We allowed them to meet and everything seemed fine. this took about a week which seemed fast to me.

After a couple days from that we had our new cat, Muska, neutered (Our older cat is also neutered) and ever since then his disposition changed, he seems to wanna play all the time but his playing involves a lot of nipping and biting, he also started meowing in a way that sounded like a chirp whenever Pazu wasn't in sight, he constantly follows Pazu also. Not only that but tries to follow him to the bathroom and also tries to eat all the food for a while he would scarf down all the food. Now we have to keep them separated. Originally they were playing fine and seemed to relax together just fine but now its a whole different story.

Side note, we do have seperate bathrooms and food bowls, we live in a 3 bedroom apartment, spacious but not enough space for 3 bathrooms or bi-level shenanigans.

Pazu is now constantly perched high, doesn't want to eat, no longer greets us, and although still friendly and likes to be petted he no longer comes down to get it, no longer even takes to treats. We were able to train Muska to sort of stop biting us, but he still tries to bite Pazu's nape whenever he is around. I'm not sure what to do at this point, Im close to sending Muska to the shelter because its too expensive to get a behavior analysis and we tried mostly everything from all the sites and nothing seems to stick. My partner doesn't want to but I'm starting to grow wary but at the same time, I know that the way he acts now will end up getting him euthanized due to his biting and playing. To add to it, he also no longer allows for petting until he is completely tuckered out from all the spazzing out and chasing our other cat.

We have cat trees, scratch posts, perches, and a lot space for them to jump up and down unhindered. We play with them both although Pazu no longer really plays much either. Muska, only uses the scratching posts, and acts like a normal rambunctious cat when alone although he bites my partner a lot while he doesn't bite me at all. For reference my partner is definitely more loving with both cats.

Ask me anything for additional info as I might be missing stuff but figured this was long enough as it is

TLDR: Got a new cat, new cat is a dick to my resident cat. Getting desperate