I think that Jason summed things up best in the initial comment in this thread when he stated,
your eyes are bigger than your stomach. While your stocking levels and/or bio load do not currently seem high to me in your 180, that is going to change as all of those fish gain some size to them. Add to that elevated aggression levels & it's a sure fire recipe for disaster.
IMO your problem isn't large water changes, it's the constant adding & subtracting of fish. It seems like you add new fish (without proper quarantine) on almost a weekly basis - and you wonder why you have health issues in some of your fish? Doh!
Below is a recent comment that you posted .....
i usually switch fish from my 180 , everyfish in the bowfront has gotten really beaten up by my red devil from the 180 but i just love my red devil to much to get rid of his nasty mean as$
If that's the case, then keep a few of the tank mates that he gets along with (at least for now), and sell the rest off. The last thing I'd worry about causing stress would be large water changes, the stress in your tank is from aggro males that are aggressive by nature.
The sooner you face that reality & begin to understand your limitations the better off you & your fish will be.
Discus keepers perform large water changes, but they do this every few days, some every day, so water parameters are always pretty constant. I perform large weekly water changes, as much as 75% at times, but my bio load is very small compared to the water volume, so my parameters are always constant, even when performing massive weekly water changes.
The only person that can say with any certainty how stable your water is between water changes is you. Buy a quality test kit & check some of the various values just before & shortly after your 75% water change. If there's a shift in pH values, etc, then try doing smaller changes twice a week as suggested. If the values aren't bouncing all over the place, then I wouldn't worry about it.
What I
would worry about is the amount and types of fish that you are forcing to co-exist in a 180 gallon glass box.