One could never have enough filtration...
Many suggest having a filter with 4-5 times GPH of your aquarium size.
More filters: more media, more bacteria, ability to support larger stock and redundancy (one goes bad, second is still up and running).
I'm sure there are tons of other pros of having multiple filters!
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?
: Also, I'd suggest to invest in good UPS. Power goes off for an hour or so, your filter keeps running...