I run dual AC50's on my 20Long & have had great success using one for mech & one for Bio. I know nutcase is gonna crucify me on this one, but it works very well in my experience... First off, my powerhead is shooting across the back wall from right to left. The water passes over the heater, then into range of the first AC50 (mech).
AC50 #1---- has no prefilter & is loaded with 2 sponges, then the stock AC50 biomax bag. In theory, this is the 3-stage workhorse & could handle the entire tank if necessary & it gets cleaned weekly.
AC50 #2----In the far left corner of the tank, I have another AC50 for bio. This one has a prefilter to keep any leftover gunk out (AC50 sponge cut in half), then is loaded with one sponge & an AC70 biomax bag is stuffed in there for max bio surface area. This media combo leaves about half of 1 space open on top of the HOB, which I find convenient for adding polishing cloth or carbon or a water-softener pillow without having to pull power or remove the carrier. Since its prefiltered, the polish cloth lasts a long time & the water softener doesnt get too dirty. The prefilter is replaced & the whole filter is cleaned when flow starts to drop, which is about once every 2 months, so I know the first AC50 is doing a good job getting the gunk before it hits the prefilter.
It has been MY EXPERIENCE that this combination works very well. It may work just as well with no prefilters & 3-stage in each AC50, but this way I only have to clean one filter on any regular basis...
EDIT: Forgot one key thing... Squeeze the prefilter under tank water to get the air bubbles out before firing up the filter. The sponge can hold a lot of air...
