Bill, you are sure going to lose CO2 from that filter setup. (Your scenario is pretty much like the overflow from the main tank for a sump filter. Reduce the distance from the overhead unit to the surface of the tank water will help in reducing the loss.) How much you can get away from the loss depends alot on the efficiency of the CO2 reactor unit. A lousy unit is not going to cut it though.

Regards
Peter Gwee