Why are so many people listing more then one on there systems? Are they not rated correctly? Meaning if a canister is rated for "up to" 90g are they not able to handle a 90g planted tank?
My thoughts on filtration:
Filtration's job is to buy you time between water changes.
Remember GPH is typically calculated using clean or no filter media.
One can NOT have more/larger bacterial colony than food source to feed it.
5-10Xs water water turn over rates is important in heavily stocked Cichlid or Gold fish tanks, filtration wise, that level is just not needed in tank that is full of plants and a few fish/shrimp.
A heavily planted tank is a better filter, than the filter is.
The water flow from the filter is arguably more important the filtration that unit provides.
Have two filters running so one always has a back-up has merit. I feel buying a very good quality filter that won't fail has more merit.
In larger footprint tanks 48"+ I feel a filter and a circulation pump or air stone is just as effective as two filters.
Filters that are hard to clean...don't get cleaned. (or certainly not often enough)