The question is why is there a dramatic change between your tank and tap water? And what do you consider a dramatic difference exactly? Some factors have an effect, others have 0 effect.
I've been doing 50-60% water changes on all my tanks for like eternity without any negative effects on long term well being.
I'd say the more you change, the more often you change, the less likely you'll cause any stress because there won't be much difference between tank water and new water in terms of stats. It's more likely identical, the tank water being richer in organics and nitrates generally.
Stuff like high co2 levels in tap water and the resulting lower ph coming out of the tap as in my case has no effect on fish health and doesn't stress them. There's a difference of PH of nearly 1 point between tap and tank because of that but the CO2 outgasses very fast. Obviously the Kh, Gh and TDS remain constant and the only parameter slightly increasing at the end of the week is TDS, which is obvious because of pollution.
If you only do the one big water change in a blue moon, then that's a different story as water will always differ between tank and tap and changing the water may cause swings stressing the fish.
The point is, fish are healthier when the tank water is not left alone to a state where it's chemical composition changes dramatically over time. That's what causes major stress in fish, not the large water changes.
