Part of the answer has to do with the extent to which solid wastes become trapped inside the filter (i.e., mechanical filtration). Mech filtration is most helpful when you export the waste BEFORE it is broken down by bacteria to become dissolved compounds and nitrates. If you have a canister filter that takes in solid waste, and then you don't clean the filter for months, all that crud will break down and become pollution in your water. Your water may LOOK cleaner, because stuff isn't floating around, but the levels of nitrates and other pollutants will rise.
There are a couple of solutions. One easy solution is to prefilter your canisters by using a sponge of some sort over the intake. The sponge can be easily cleaned every week (or less, if you prefer) to export the crud before it breaks down.
Another easy solution to separate filtration tasks. Have a prefiltered canister packed as bio-only (i.e., loaded with ceramic noodles and/or bioballs, and nothing else) to provide biofiltration and a second filter, preferably one with easy-to-clean and durable media, to provide mechanical filtration. The bio-filter canister can go a very long time (months, at least) without being cleaned, and the mech filter media can be cleaned regularly.
For a 20, if you were to use only your single filter, a prefilter sponge will go a LONG way to keep the inside of the canister from loading up with crud that will break down into pollution.
There are several routes to prefilter, none of which are very expensive. If you're interested, just ask.
HTH,
Jim