You can get a carpet of DHG (dwarf hair grass) to grow in medium light with no Co2. I have some growing in my 54g that has medium light and no CO2. The carpet was much more dense right now it's nothing like in my 40B, largely because other plants have overgrown it. I should do an experiment in that tank and plant a section to see how it grows there compared to my other tank.
Others will have to chime in as I am not the expert with this planted stuff....