There could be a valid reason why one, or more, of your cats fight in the home, although determining any of these can be difficult for the owners concerned.
A possible reason could be that one of your cats may have a funny or odd "smell".
It is natural for all of us to have a particular smell or odour.
Cats, and other animals, are no different in that respect.
Your fighting cat may not recognise, or accept, the smell of the newer cat and may see this as a sign of rivalry or a threat.
As cats are naturally territorial (but also inquisitive!) an odd smell can have the effect of the first cat becoming aggressive and defensive.
Luckily, with time, cats often accept their "intruders" and many cats can become close and friendly if they live in the same home environment.
Some owners have advocated a novel way of clearing up the "smell" issue.
The owner should get a clean towel and rub it over the newer cat.
Then, the same towel should be used to rub over the older, established, cat.
The idea is that both of them will smell very much the same and they will come to accept the smell as part of either them or the home itself.
It is not inevitable that cats fight and many owners have witnessed the introduction of alien cats without any problem whatsoever.
Much can depend on the individual personality of the cat(s) as much as anything else.
Indeed, I have had a cat which took to any foreign cat it ever met.
Persevering with the issue can pay dividends.
Using the towel method to coat each cat in th same odour, if it works, will lead to a lessened need for this process as time goes by and ultimately the practice can be stopped altogether.