I have two perfectly functioning Fluval 404/405 that are currently on my project table. I can tell you that in their stock configuration they are probubly the worst designed filter with respect to basket and sponge media configuration design that I have every seen, bypassing up to 60% of unfiltered water strait through the canister. Even the impeller housing is designed for by pass.
My 1st Modification was to create easy connect adapters to the impeller housing and canister body to allow the use of standard 12x5 inch pleated micron cart filters which drastically increased effective filtration considerably. However as simple and effective as that mod was I found a micron polisher requires much more frequent bleach baths every 4 to 6 weeks, in addition a micron polisher on the floor rather then as an easy accessible HOB (Mag 250) made frequent cleaning more of a hassle then a help IMO. I want at least 3 to 5 months with 75% GPH endurance with outstanding filtration performance and the black Poly batting's to prove it.
So I have been slowly piecing together my 2nd Fluval 404/405 Mod which totally eliminates the sponge and sponge slot and the entire canister minus 1 inch is completely occupied by a custom made plastic mesh basket that will be capable of holding course media on the inside and poly batting on the outside 360 degrees, top and bottom. Similar in concept to the Magnum 350 but far larger, effective, and simple to use. The impeller housing will also have a much better ring seal that will prevent bypass around the impeller housing. In the end plastic mesh baskets will simple screw on and off for chnage out replacement and Poly-batting change out without the whole "hazardous waste cart bleach-chem-bath routine" the repeated exposure of which to my lungs hands and face membranes on a routine bases has gotten really old even with the with the Mag 250. Sounds silly but as a former level 1 HAZMAT responder and coordinator I can tell you that this type of hands on physical exposure to bleach is not good, sooner or later you run a much higher risk of just 1 cell mutation followed by cancer, then when not exposed on a routine bases. Anyway that will be finished behind a few other projects I'm currently juggling.
OH and to the PO I say, if the convenient and familiar use of stacked basket canister filters is most important to you, then the Sunsun HW302 is your top filter.