OK - I just bought a Penn Plax Cascade 1000 for my 75 gal tank, and I have ZERO experience with canister filters. I've read quite a bit about them, but I'm still fairly lost. Here is the way I set mine up today - please let me know what you think. I couldn't tell much difference in the pads that came with the filter, except for the obviously different black pad.
Bottom - tray 1 the bottom of the filter has this coarse black pad (came with the filter)
and I put this in the tray with it: this is some stuff I bought today that looked like "medium"
End tray 1
Tray 2 - bottom of tray has another layer of this blue stuff like the top of tray 1, then this - one of the pads that came with the filter
End tray 2
Start tray 3 - full tray of bio balls
End tray 3
Start tray 4 - another one of the pads that came with the filter - on top of this is a bag of carbon that also came with the filter
End Tray 4
Start Tray 5 - Filled it with this stuff - which I think is a polishing layer??
End of all trays..
I basically tried to go from coarsest foam type pad to the finest foam type pad, with bio balls near the top and the carbon bag the last thing before it hits the "polishing" pad - if that's actually what it is. It makes sense to me to start with the biggest holes and filter down to the smallest - beyond that, it's purely another language to me. I'm trying to wrap my head around where the bio balls and/or ceramic pellets and/or carbon stuff all come into play. Unfortunately, I don't know of any aquarium shops around here that I can rely on - unless you count a pet mega-store... LOL