No, you don't have to. Enough detritus and stuff will be swept into the fuge from the main system. It'll be more than enough for those critters to stay happy.
For FW? You really don't have the diversity of microfauna in FW to make it worth it. I suppose you could have a separate unit filled with plants for nutrient export, but if you're going to go through that trouble, just make a planted tank. FW plants are too beautiful to squirrel away out of sight.