Keep topping of at the same level you start off at with water,the concentration of your solution becomes stronger as the water level drops,see water is what evaporates and gets consumed,of course nutrients also but not as fast as the water does.If you check your ec/tds levels you will see when your buckets are low the ppms will increase drastically and can cause nute burn and make your girls unhappy...,..,.,.,,,.,.,.,.,.,.,.Add nutrients also if you top an your tds/ec level is not at the desired levels.Moving buckets is one of the reasons I stay away from dwc.Here's an idea,What if you get an extra empty bucket and replace with the 1 you are changing 1 by 1??That way you have somewhere to put your plant while changing the reservoir.I know it's labor intensive but hey you gotta do what you gotta do.