That's the problem with the dang things...canisters in general, I mean. They are such a PITA to clean that
nobody does it as often as it really should be done...so that crap just accumulates inside for however many months
or years 
WHOA

and bacteria has its way with all of it.
I'm not sure if there is an actual official definition of "nitrate factory"...but I'm pretty sure that this would fit if there were. You are sweeping dirt under the rug...and doing it for so long that the pile is starting to become a tripping hazard!
Yep, your massive water change regimen makes your tanks work beautifully...but that's
in spite of your skunky canisters, not
because of them.
Canisters suck.
A venturi tube attached to a powerhead (most of them seem to offer this option) results in an outstanding aerator/agitator, at least as good as a canister...without subjecting you to the potential problems of the canister, i.e. potential leaks at O-rings, unnecessary hoses flopping around, etc. Any impeller requires periodic cleaning/maintenance; every time you do so you risk causing a leak with a canister. Plus, compare the flow rate in gph of a canister with that of a powerhead of the same wattage; they're not even close.
I'm a big fan of reduce/re-use/recycle as well. If somebody gave me a canister, I'd probably figure out some use for it...not sure what it would be...oh, I know: I'd give it to somebody else!