CichlidFish.com - Fish That Think.

is it possible to do too many water changes?

Depends on how you do them and what kind of fish.

its an oscar. is 1/3 every 2-3 days too much?

You have to remember that in the wild their water is changing all the time. Just don't go overboard. And IMO you're not overboard. As long as its just the water that your changing and not gravel vaccuuming that often or changing filter stuff that often.