10-13-2010, 03:56 PM
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I think the main reason is the more advanced filtration since you have more filter media options and most canisters let you customize what you put into it. They also allow for larger surfaces for bio filtration, foam used in canisters are much larger than HOBs. There are smaller canister filters available for smaller tanks, its just far more convenient to use HOBs or internals for small tanks so demand has not been as high for canisters in small tanks.
Also canisters afford you more placement options for your filter< ie tanks that need to be against a wall will probably be better with canisters so the filter is underneath rather than HOBs which will cause a large gap between the wall and the tank.