As long as you make sure the levels(pH, temp) of the water you are adding are the same as that of your tank, then it should not be stressful to the inhabitants. The waste that is picked up by your protein skimmer will not include significant removal of nitrates, so you should continue to do regular water changes. I would suggest changing about 5 gallons every two weeks. Be sure you aerate the water you are going to add for a few hours. Just thought of this, but you are using purified water, correct? Not tap water, I hope. As most have found, tap/well water contain far too many contaminants to be used for such delicate creatures. HTH