My Amphilophus is in a 125 by himself, gets fed sparingly compared to how many hobbyists feed, and gets an 80-90% water change weekly. With my tap water, and my set up, this is not an issue. He’s almost 15 yrs old. Might just be good genes, or good luck, lots of TLC, or a combo of all of the above.
Our fish swim in their own toilet, the more you flush, the better it will be for the fish.