misty; I use mainly Emperors on my 10 tanks and love the way they work. As far as the bags I change them when the water level gets to the top of the bag. I do not believe in re using dirty bags (altho my son does rinse and re use his- we argue that all the time). The bags are not cheap as you know but I have found a place that sells them for 8 bucks a 4 pack (and that is the cheapest I have seen) which translates to about 4 bucks to change both which is cheap money to have a clean, healthy, happy tank. I do find tho in a tank with algae the inlets have to be cleaned more often as the openings are somewhat small(that is my only gripe with the Emperors). I even use one on my 58 Reef tank and it has done fine for over a year.