It depends on what types of coral you keep but I like to see marine tanks cycled a minimum of 10 times an hour, so at least 240 gph for you. A single Koralia nano provides that much flow. I include skimmers/filters and powerheads in this as they all provide flow. I have a 40 gallon SPS holding tank that I use a RIO+ 2500 on at 3 ft head, so I have a flow of 592 gph (all filters/skimmers are in sump) on that tank and everything does well. I also use a couple mini powerheads to provide more turbulent flow in that tank.