New and in need of some advice

Hi, I have been a keeper of malawi for gd few yrs now and I am planning on changing over to marine. Want to start with a FOWLR set up and then venture into reef. I will starting with my fluval 20g and wanted help and advice with filtration. I currently have a fluval 204 cannister and ideally would like to continue using this but what recommendations do people have with regards to media in the baskets( sponges, carbon, alfagrog, bio balls, rings, phostgo). I have read a lot about how cannister are nitrate factories, but I have kept overstocked malawi tanks for yrs and found with regular once a wk water change and filter cleaning kept my nitrates down. If I follow the same routine surely that would keep nitrates down so using a cannister wouldnt be an issue or would I still struggle? And if this helps I only plan on keeping around 4/5 fish in tank something like clown fish etc.



Any help will be appreciated.