[whatcaneyedo]

How large a system(s) are we talking? A few little tanks off of one sump makes more sense to me. You'll probably benefit more by having them all more stable than if you keep them separate. But if you intend on having large tubs containing hundreds of gallons each you probably want to keep them separate and specialized. That is the conclusion most people seem to come to. Small frag tanks running off of the main display and sump are pretty common. But in commercial green house style operations several independent systems seem to be the norm.