did your hairgrass go through a phase with algae on it? How do you keep algae out of the tank? I have a 2.5g with 27w +co2 +excel, though get regular green algae on glass and hairgrass. does this eventually end? the tank is fairly new, set up for a few weeks now.Excel helps in combination with CO2.
When used alone, it is not as effective as CO2, but nevertheless provides some form of usable carbon for plants--it's not CO2 so it won't gas out.
If you had to choose only one, CO2 would be better. But excel with CO2 is a good choice. CO2 would be the bread, excel would be the butter. You don't have to do excel, but it doesn't hurt.
I grow dwarf hairgrass in my 2.5g with 27w + only CO2, and they're doing really well. But I haven't tried excel (too expensive for me) so I can't really say from personal experience.