Just top it off with half-strength nute solution.

Now, question - why two weeks? I've done it before but usually a weekly change is the best idea.
khyberkitsune Reviewed by khyberkitsune on . topping off hydro unit When you grow hydroponically(DWC), you have to keep your reservoir topped off correct? Do you just keep it topped off for the two weeks with fresh water? Or do you have to add nutes to all the water added during the two weeks before water change. Rating: 5