Not changing water, or not changing enough water regularly, will stress out fish much more than doing a large water change. This is because the more water that is regularly changed at one time, the more stable the tank's biological system becomes, and that means less stress. Any deterioration in water quality, or any fluctuations in parameters, cause stress. Larger regular partial water changes minimize fluctuations, so less stress. And this means a healthier physiology and a better functioning metabolism.
