The total numbers of bacteria in the tank, including the filter, are only up to the job of converting the waste produced by the current fishload.
If you remove a good proportion of them, you are going to have a deficit for a while whilst the filter reseeds.
There is an easy way to test whether this is the problem - test for ammonia and nitrite. If either of these is present, then you have a biofiltration deficit. This might also make the smell - hence my suspicions in this area.
What sort of cartridge regime you should adopt I can't say, not knowing the filter in question. If you post the make and model, others who do know will be able to help. We don't tend to have HOB filters here, but other US posters would be able to help, I imagine. I would suggest, however, that you should rinse the cartridge out - run tank water through it in the reverse direction to clean it, when it gets dirty.
If you remove a good proportion of them, you are going to have a deficit for a while whilst the filter reseeds.
There is an easy way to test whether this is the problem - test for ammonia and nitrite. If either of these is present, then you have a biofiltration deficit. This might also make the smell - hence my suspicions in this area.
What sort of cartridge regime you should adopt I can't say, not knowing the filter in question. If you post the make and model, others who do know will be able to help. We don't tend to have HOB filters here, but other US posters would be able to help, I imagine. I would suggest, however, that you should rinse the cartridge out - run tank water through it in the reverse direction to clean it, when it gets dirty.