I do water changes every week (about 15-20%), but I'm a little anal about water clarity and my protein skimmer is too small for my tank at the moment. Most people do every other week. Either way you'll be fine, you'll just really have to find what works for you. If you notice your parameters getting out of whack, do a water change. I rarely do top offs, just do a water change since by the end of the week when the top off is needed, we're ready to do a water change anyway. Top offs never worked for me and always through off my salinity readings.