It seems to me that there is a lot of confusion about filtration. Not the least of which is the silly notion that we need 4-10 times the aquarium size in gallons per hour (GPH) flow rate. More water flow through filter media does not make for better filtration. Better filtration only occurs when we filter water [often more slowly] through finer media. So, it's not about how much water we pump around, but how well we filter the water that we do. Think of it...if the water is well filtered, filtering every drop twice per hour would seem to be plenty...right? Skeptic? Okay, lets say 4 times per hour. So if we filter every drop every 15 minutes and don't clean the water...pumping it faster probably isn't gonna help
I had my filter setup with the standard sponge and bio-media and I was noticing tiny flects of detritus in the return water. Particles like you see when you use a gravel siphon. After I added some filter floss to polish the water in the final stage, the problem was solved.
Now one would think that multiple filters on a small tank just doesn't make sense. But then as I thought of this, I was pondering my 29h tank and wondered...is two filters rated for 5-15g really any different than one filter rated for a 30g?
Although larger, another example is my 60g. When I bought the 60g I purchased the AquaClear 70 HOB. I really like the AquaClear design because in contrast to cartridge HOB filters, I/we can adjust the media type, order and volume. I was happy with the filter, but wanted a bit more media capacity. I guess I could have purchased a canister and shelved the AC70, but instead I ordered another AC70 filter. One filter has sponge, Seachem Purigen and filter floss (mechanical). The other AC70 is filled with Seachem Matrix and uses a Fluval Edge sponge pre-filter on the inlet tube (biological). Both filters are set for low flow / maximum re-filtration and use simple DIY water bottle baffles to further reduce return water flow.
I also hear a lot about folks adding more filters and/or powerheads because some mulm is collecting on the substrate. I also hear about folks with canister filters adding a HOB.
Especially if you have a planted tank, the mulm might better be allowed to decay and help naturally feed the plants (can you say organic aqua gardening?). Even in the unplanted tank, this material does not hurt anything and is easily removed with the weekly water change/gravel siphon.
Speaking of it, not to overstate the obvious, but the WWC is the very best 'filtration' as 'the solution to pollution is dilution'. Some 'stuff' just can't be filtered out. So in some cases, multiple filters, regardless of tank size, may make good sense...in other cases, maybe not so much.