Canisters can be nitrate traps if they are not cleaned regularly. That being said when I ran two fluval 405's on a 55g I cleaned them and replaced the media once a month. If I went too long I would start to have problems with algae sometimes.
I used a source of carbon and used the bio pellets. Once in awhile I'd throw some gfo in bag in there as well.
I have a fluval fx6 up yo 1500 litre on a 450 litre tank.. This is the filter I'm gonna use along with live rock and a protein skimmer. So u recon every month should be enough?