TWO AC200 or 1 canister ?

The answer to your question would depend on which canister you want to compare to the AC200s. Some canisters wouldn't be equal, some would exceed the AC200s.

What size tank? What fish? How many?

Jim
 
I would have to say for trapping fish waste, a canister is better b/c of design, but the difference is very small.
Biological capacity would go to the filter with the most media, most likely the canister.
Crystal clear water is a toss up...either.

In your case, I would go with the AC200's for maintenance reasons, they're easier to clean.

The "problem" with HOB like the AC's is the tendancy of the foam (media) to float when full of waste and allow water to bypass the media. This isn't a problem if you maintain them regularly though. Generally, canisters hold more media and don't allow for any water to bypass the media.
 
depends on what kind of canister.
 
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