I believe the Marina manufacturers - Hagen - advise changing 1-2 of the cartridges every 2-4 weeks which would work out quite expensive and not particularly good for any friendly bacteria that might have developed as you would be throwing some if not most of their habitat away. As fluttermoth suggested, replacing the cartridges with foam might be the best way forward even if it means starting again. Foam only needs to be thoroughly rinsed in the removed old tank water when doing a water change once the tank is established and doesn't need replacing until it is literally falling apart.
I know it is disheartening but as you are only in the early stages, it will be well worth it in the long run.
I know it is disheartening but as you are only in the early stages, it will be well worth it in the long run.