No matter how you cut it horizontal or vertical scrubbers end up with water spritz hitting everything around. Those lead to growth where you don't want it. You also always have some noise from rapidly flowing water. You need a pump - somewhere. As for the algae you have that drat boundary layer.

A wheel style would require no pump anywhere.
No splashing.
No noise.
And the major point would be elimination of the boundary layer as the surface would briefly "drain down" every rotation.

rygh; The CO2 is dissolved as a gas in the water. This is where the algae extracts it from. Not across the flowing barrier the water presents. The higher levels of CO2 would be from the life processes in the tank not in the surrounding air. CO2 would actually be transported out of the tank water into the air on the rapidly moving surface flow.

One RPM would probably work OK. There are many gear motors with crazy torque available with 1RPM - (think second hand).
http://cgi.ebay.com/SYNCHRON-1RPM-Motor ... 20af55015f