all this sounds awfully familiar to those doing product design and development. we should identify the needs first and apply the
Kano Model
Needs
(1) Biological filtration - keep fish from being poisoned - (Threshold feature - customer doesn't really care how its done whether by HOB or canister, as long as its done))
(2) Mechanical filtration - keep water clear to enjoy the tank more (Threshold feature -customer doesn't really care how its done whether by HOB or canister, as long as its done)
(3) Don't like cleaning filters - idea of a perfect canister filter is one that is never opened. (if product has this feature it will delights the customer)
Customer needs 2 threshold features to be satisfied but will be truly delighted only if he never needs to open up the canister filter.
I guess that led to the development of the Oase Biomaster with separate removable prefilter. The Fluval 07 series went a different direction and decided on a super-huge prefilter compartment that can take in a lot of dirt before it ever gets clogged, so you don't have to clean it as often. Each has pros and cons, the Oase has a huge footprint to accommodate its various features, the Fluval 07 is much more compact. Customer has to choose - easy to remove prefilter that has to be cleaned a bit more regularly since its smaller, or a huge prefilter where you still have to open the canister, but the interval is longer.