It says to do the water change when you are done dosing it.I just do a 15 to 20% water change every week.I would dose it through the week,and then do water changes on the week end.Oh and take into account the amount of live rock you have in your tank.Because lets say you have a 30 gallon aquarium,you do not have 30 gallons of water in it,because of the liverock.Just saying so you do not over dose it.