I don't have fluval. I switched to Eheim from Mags.
It is just an empty can in there (green empty can so does not grow crud... nice_.
It just has bottom and top plates for nice flow.
So:
Media, put what you want in there. For example I am curing some Live Rock atm so I took 1/4 pound or so of reef carbon and stuffed it in my moms old nylon socks (have a whole bag of ladys old nylons). nylons are great because they will fit right to the size of your can with whatever you put in it.
Example
bottom plate
filter screen
nylon bag of Ehiem donut rocks? forgot the name of those things.
filter screen
nylon bag of carbon
filter screen (smaller type)
nylon bag of amonia carbon stuff or bio balls or whatever is your deal)
cover plate
when it comes to cleaning this stuff the nylon bags are easy to pick up and rinse and put back in or chuck away. So start bugging the women in your family to save ruined but unsoiled socks
IMO if I was to do a can I would NOT put filter crap in there. just put a few nylon bags of those donut things. Get or DYI a good prefilter that is easy to remove from the tank and rinse oten. I would rather do the mechanical filteration where I can reach it and let the biological filteration happen in the can (where I don't have to clean it). I do heavy prefiltering and after one year my can was clean as a whistle. When I rinsed it out NO detrius was visable.
....
Recently I took down my sick tank for moving stuff around. I took the 20 buck penguin 170? and put it on the back of the main tank. Now I just leave it there. the cool thing is if I needed to set up the sick tank it will still have most of the good bacteria in it. Also it makes a nice place to add charcole if needed. (I have never needed it ... but hey you never know when some kid (not that I have one) will dump some toxin in there... any how, you get the point right... fault tolerance.
(I keep a spare $8 10g under the stand).
HTH