FM,

Most of the time, we do not advocate doing 100% water changes, because the new water could be totally diff. from the old water, in areas like dissolved O2, CO2, pH, KH, GH, temp., and lots of other stuff that your fish would have to get accustomed to. During this period, they will be under a certain amount of stress, and if weakened too much, could succumb to infections.

A 20 - 50% water change is normally done to remove excess NH4+, NO2- and NO3- ions from tanks that may not have efficient bio-filters. Usually it's those hobbyists that use only mechanical filters that change so much water.