In my experience, fish really benefit from clean water, so there is nothing like too many water changes. What creates a problem for fish is rapid changes in the water quality. If your fish are used to a 20% wc weekly and you do a 75% one week, they will probably freak out. However, fish could easily adapt themselves to these changes if they are progressive : two weeks at 20%; two weeks at 25%; two weeks at 30% ... two weeks at 75%. When adapted, they clearly benefit from the clean water (low nitrates, hormones, polluants); live longer, grow faster and bigger, more colorful...
It is the same principle when you add new fish in a tank that has a 30 ppm nitrates level : the old fish seem fine since they are used to it (progressive increase of nitrates level) while the new ones are stressed and may get sick.
This scientific study even found that fish could get used to ammonia. Here is the abstract :
"An automatic flow-through dosing apparatus was used to determine the effect on Tilapia aurea of acute and chronic exposure to un-ionized ammonia. For fish not exposed to ammonia prior to acute testing, the 48-hour median lethal concentration (LC50) was 2.40 mg/liter un-ionized ammonia. After fish were exposed to sublethal concentrations of un-ionized ammonia (0.43-0.53 mg/liter) for 35 days, concentrations as high as 3.4 mg/liter caused no mortalities within 48 hours. Histopathological changes occurred in the gills of fish given both nonlethal and acute doses of ammonia. Capillary congestion, hemorrhaging, and telangiectasis were common symptoms of gill abnormalities".
Source :
Acclimation to Ammonia by Tilapia aurea
BARRY D. REDNER and ROBERT R. STICKNEY Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, Texas 77843