Is your tank cycled? Once a week is usually good for most cycled tanks. It really depends on how high your nitrates get in a week's time, and how much water you are changing each week. I do about 50% on each of my tanks weekly.
Weekly water changes are not to much. Keep it up! Water changing has no adverse affects on the fish. Remeber in a tank they have to live in what they eat (or what happens after they eat). Where they live in the wild is quite different from that. How much do you change?