If you do have varying parameters in your tanks for whatever reason or that vary from the tap water, I think it's better to do more frequent smaller changes. You could do two 30% changes a week rather than one 75%. I do one 25% a week on all four of my tanks and never had a problem since I've been doing that. The other factor is that I don't have extra heaters etc and don't have space or time to make sure I'm equalizing the temperature exactly every time. If you do a 75% change, you need to make sure the temperature matches or is at least pretty close. If you do a smaller change, you can ballpark the temperature and it's not as big a deal. Just my two cents.When you change 75% of your water, how do control your pH. Or do all your tanks just run at the pH from the tap, which means that most of the systems will be Alkaline. Then how do you keep acid loving fish or are they just something you don't consider.