Oscar Aggression

[mahaffeywj1984]

I have a 5 in or so Oscar, I posted about him a few weeks ago when he had larger tankmates huddling in the corner. I got him a 90-gallon tank to get him out of the 55 gallon. I also decided that I wanted to try him out with a bigger tank mate in it. So, I choose a gold severum that I've had for 3 years. He was about the only one from the initial fiasco that was able to venture out of the corner of the tank any distance. I also added several younger silver dollars large enough not to be eaten and small enough to not overload the filtration or take up too much space. In hopes that they would see the aggression first if any were to present. I added a lot of drift wood and taller plants to create line of sight breaks everywhere. I then added both the oscar and sev at the same time. Everything went great for a week or so, except for a little chasing when feeding but nothing crazy. Then this past Saturday I did my morning check and found the sev floating vetically face down but alive. The SDs were all hiding under driftwood and plants and the Oscar was swimming happily at the glass waiting for food. The sev ended up passing a few hours later in a hospital tank and now the Oscar is in isolation all by his lonesome. The only thing I can figure is that the sev and oscar got into it and he got a damaged swim bladder. I only saw a red mark on the sevs side. I did check water, pH was 7.1, same level both had been thriving and surviving in when separate, no ammonia, no nitrite, and nitrates were around 40. I did do a 50% water change just in case something funky was happening. Initially I had planned on upgrading everyone to a 180 and combining, am I looking at a wet pet now or would having the extra space be ok. He would be going in a 150 - 180 with another 2 sevs, a chocolate cichlid, more SDs and a pictus cat. Am I just romanticizing the thought that someday I will have the "community" SA aquarium complete with an Albino Tiger Oscar? I really want an Oscar in this setup so I would have to get another one if it isn't foreseeable. I do love the one I have so I suppose he will just be chilling in his own tank if not. I love the wisdom on this site as it has been my go-to for reading anytime I am exploring anything cichlid related and I'm a big boy so I can take any harsh criticism and will heed all advice. I'm not just looking for someone to tell me what I want to hear. I'm not new to the hobby but I also don't think I'm an ichthyologist, I have a long road of learning left to travel.