I like having dual filters on my tanks 55 gallons and above with each filter having the capacity to handle the tank. The reason is not water flow but increased cleansing capacity. You can reduce the water flow rates so that the fish are comfortable. Another positive of having dual filters is that if you have a filter failure your tank is still functioning. I like having the same filters to keep the supplies, etc., the same. If this was my tank I would go with 2 FX6's.