First off, not sure what you mean by "perfect levels" for your water??? Can't imagine that your nitrates are at 0, among other things.
There is a lot more to it than water quality too.
That is a lot of constantly stressed out fish that will eventually start acting on that stress by fighting/killing other inhabitants which sounds like it is already taking place some. As the daily stress continues you are most likely shortening the lifespan for all the fish.
There is a big difference between a tank that is properly stocked with multiple fish and the setup you have. Sadly, you are probably going to end up with a tank with 1 or 2 fish in it when all is said and done, even with them all in a 120(still would be overstocked).
I strongly encourage you not to get more fish, it is just a death wish for all the inhabitants.