As in more separate units? Longer not better, due to oxygen being long gone and the biobals being the limiting factor? I am kinda lost here. How can 2 "small seperate units" be better than one super large unit with a lots of wraps of coil and a large diamter?
ok so clearly i didnt clear anything up
the bio-balls are almost never the limit, the coils supplying the bio-balls is almost always the limit(within some reason).
as soon as the O2 is removed from the water, the water *should* be outside of the coil and in a reduced flow area(but still sealed from O2). if your coils are longer it has a number of effects, mostly effeciency until you get really really long, then it gets nasty. if its short you get nitrite and nitrate and no good.
you dont need a large amount of media outside of the coils for the work to be done, so if your in design mode build 1 unit with 2 or more coils, if you already have a unit then either upgrading (tearing the one you have apart and adding more coils) or adding an additional unit.
even when you get longer larger diameter coils your still better with multiple coils, its still an effeciency thing.