Just add septico..... Use a safe septic tank bacteria.....no clogging. I use 1 teaspoon of septico whisked then everything including the undesolved stuff (which I assume is food for the bacteria) I between your first and second stage mech filter) filtration..Since the comments have turned to exposing the downside of canister filters, I'll offer my reason for abandoning them years ago in favor of sump filters. I have run many brands over my first 25 years in the hobby. Eheim, Nu-Clear, Ocean Clear, Marineland. The problem with all of them was the same. As the filter media loads up, flow drops precipitously. You clean the filter and you're running 1200 gph, then a week later the media clogs and you're only pushing 200 gph. This is terrible for conditions in the tank. Cleaning is a pain, especially when pleated filter cartridges are involved. With a sump filter, the volume of media can be huge. Flow never changes, no matter what happens to the media. There are so many ways to set up a sump filter that the cleaning cycle is measured in months, at a minimum, instead of weeks. I'll never go back to a canister filter.
Ultimately we all know the sump is the best way to go.... I have a 32 litre betta tank with a sump....Good advice from several on protecting the main filter media with a prefilter. Not trying to convince the OP to go with a sump over a canister filter since we are only talking about a 40 gallon tank. Would not bother with a sump on that small of a tank. My preference for sump filtration is based on the size of my tanks (300-900 gallons) and my insistence on not seeing any equipment, tubing, heaters or anything else unnatural in my aquarium. Also agree with the advice to sacrifice, if necessary, on the front end to get a quality filter. Will save you lots of frustration and perhaps your livestock.