I dont have the money to get my sump a nice light/substrate or more plants but once I do, Im planning on putting lots of plants in one side. Then I will run the lights on a timer on an opposite schedule as my main tank. That way the sump makes o2 for the fish at night and when the sump lights are off during the day, the plants will make co2 for the main tank. wont be much, but any co2 it adds will be better than without it and its free. Plus a great spot to grow out some plants or hold some extra fish. then use the other side of the sump for mechanical filtration.
search freshwater RDP sump if your interested in trying it. Im sold on it, just need to buy a light!