Never had success with dwarf hair grass (eleocharis parvula) without pressurized CO2 (I define success as plant is growing/carpeting/propagating itself) . Some people like to use excel as a CO2 replacement, but in my opinion it doesn't work (some people love it though, never worked for me).
Excel is essentially diluted glutaraldehyde. In my opinion, It serves mainly as a sort of algaecide, which essentially gets rid of algae competing with your plants, while it can provide some carbon source for your plants, you get about the same amount 2-5 ppm from normal atmospheric gas exchange depending on water flow, kh, ph.
You can change the environment to fit your plants, or change your plants(species) to fit your environment. Personally I think a photo-period of 4 hours of light is too little, and would increase to at least 6. If there is a bloom of algae, then consider nutrients/co2 balance may be off.