Total bucket capacity of fx5 is more than 5.9, 5.9 is the bit they leave for you to decide what you put in there,
if you so wished you could remove the sponge surrounds and put what you like in! I wouldn't do this myself though. Happy with the 5l of eifsubstrat iv'e just popped in plus 1 polishing pad. Mine arrived this AM

Build quality is superb, only problem is that I got my measurments wrong and it wont fit in my cabinet, no worries it sits fine to one side. excellent arrangement of baskets, lifting handles, The whole quality of this filter is WOW, the strap bolts on the lid are kinda never ever gonna break! it's super solid, easy to clean, you can drag any excess debry out of the filter through the purge valve, cool. Clever Smart pump technology, removes any air by itself and includes thermal overheat protection in the case of a severe blockage, pump shuts down untill cooled off!
And I cant believe it's so damn quiet!. I didn't use the fancy hose holders for the entrance to my tank, It would mean me destroying my hood, but the hoses fit perfect through the exhisting holes in my hood and I had some rung suction cups that hold everything inplace. So far I am delighted with this filter it's more than I expected it to be and anyone thinking of buying the fx 5 wont be disapointed!
If you want to see the fluval fx5 close up all parts, baskets etc.., check out the Manual it's a PDF file so you need Adobe Reader installed, here's the link:
http://www.hagen.com/pdf/aquatic/FX5_user_manual_EN.pdf
I will take some pics too, and keep you posted on any happenings Good or Bad.
Cheers