In　marine　recirculating　aquaculture　systems　(RAS),　the　prolonged　initiation　of　biological　nitrification　and　the　proliferation　of　potential　pathogens　due　to　high　salinity　pose　challenges.　A　combined　biofiltration　system　(ceramic　rings　(CR)　-　granular　activated　carbon　(GAC)　-　ultrafiltration　(UF)　process)　was　implemented　in　an　actual　marine　RAS　for　selenotoca　multifasciata.　The　research　spans　200　days,　focusing　on　ammonia　nitrogen　and　dissolved　organic　matter　(DOM)　removal,　membrane　fouling,　and　microbial　communities.　Results　showed　that　fast　nitrification　start-up　achieved　by　the　CR-GAC-UF　process　within　21　days　under　low　ammonia　concentrations　with　a　daily　ammonia　removal　efficiency　of　91.7　%.　UV254　and　CODMn　maintained　at　0.05　±　0.01　cm−1　and　1.34　±　0.03　mg/L,　in　line　with　the　stable　removal　of　humic-like　substances　and　soluble　microbial　products.　UF　membranes　maintained　low　fouling　with　fluxes　at　15　LHM　and　the　protein/polysaccharide　ratios　in　extracellular　polymeric　substances　(EPS)　were　stable　below　0.5.　Specific　dominant　bacterial　genera　facilitate　DOM　removal　such　as　Xanthomarina,　Idiomarina　and　Hyphomicrobium　were　recognized.　Candidatus_Nitrosopumilus,　Nitrosomonas_sp.　and　Nitrospira　sp.　ENR4　were　detected　as　the　core　nitrifiers　in　CR-GAC-UF　process,　which　were　closely　related　to　the　amoABC,　hao　and　nxrA　genes　(p　＜　0.05).　The　multi-stage　barrier　of　CR-GAC-UF　process　maintains　low　heterotrophic　plate　counts　(3　±　1　CFU/mL)　and　reduces　potential　pathogens　in　steps.　Taken　together,　this　study　provides　a　theoretical　basis　for　the　wider　application　of　marine　RAS.　©　2024　Elsevier　Ltd