They dont nessisarily do waterchanges, if you dont have a reef, the salt content/calcium and most general water parameteres will stay the same with only minor tweaking needed, except ammonia/nitrate/nitrite/phosphorus, which can be removed with good biofiltration/ and chemical filters (or huge algea growth). Suffice to say, water changes technicaly arnt nessisary if things are done correctly (but its extreamly difficult to do in most systems), but will be a disaster if done wrong.