Water flow in a saltwater tank is vital for a lot of reasons and also depends on the type of life you stock in the tank. In a freshwater tank, I dont think its necessary to sustain life. Some freshwater fish dont like a lot of turbulence in the water at all. I think it might be best to research the type of fish you plan on keeping in a freshwater tank and determine if they prefer a lot of water flow and good circulation. Your filter should provide enough circulation to keep most fish happy. Someone with more experience in saltwater systems might be able to give you better advice as to why water flow is important in a saltwater tank vs. a freshwater tank.