I believe the G5 is rated to 800 gallons, but realistically I heard it is only really effective up to 400. The G3 is supposed to be good up to 300, but again, only effective up to 180. I will be running the G5 and my G3 on the new 300 gallon. My total system volume will be roughly 475 gallons. Hopefully that will be suffiecient.
Tom