I have an Aqua-Flow 300 filter; it has a sponge with activated carbon in one chamber, and CrystalMax in the other chamber. The maintenance instructions say...
“The dual-action filter cartridge contains activated carbon. Clean the filter cartridge every 1 or 2 weeks by rinsing it with clean tap water. Replace this filter cartridge monthly, the carbon in the filter cartridge is no longer effective after it becomes fully saturated”
This has really confused me as it goes against everything else I’ve read about filter maintenance - I thought you were meant to clean your filter about once a month in the dirty water not clean tap water, and I also thought the whole point of cycling was to build up the good bacteria in the filter - so why would you completely replace that and have to start again once a month?? 🤯
You have accurately grasped the concept of cycling and filter maintenance.
Unfortunately, the filter manufacturer hasn't / is profit-oriented and hence has this set-up/instructions.
After finding that the filter cartridge in my previous filter did indeed clog and pose problems every month or so, I changed it altogether, filling the compartment in the filter with sponge/foam only (plus as much of the filter cartridge's filter wool that I could retrieve to get the new filter sponge 'seeded'). In your case, rather than even give the filter cartridge a try, I would replace it entirely with sponge/foam.
Ah - I had a momentary memory lapse and forgot you already had fish. Yes, in that case, retain as much as you possibly can of any foam/sponge and wrap it around/stack it as close to new foam/sponge as possible to 'seed it', or, depending on shape/size, squeeze new foam/sponge into the gap where the carbon was. [There is no need for any carbon - although it can be helpful to retain in a cupboard and use following treatment with medication.] It wouldn't do any harm to add some Tetra Safestart or Dr Tim's One and Only too to the new foam/sponge too - although the evidence for effectiveness of products such as these is a bit unclear, these two are thought to be better ones.
Isn’t it terrible that the so-called people you should be able to trust - manufacturers of a product designed to keep fish - give out such bad advice - wondering if I should complain - for all the good it might do 🙁