By liquid co2 I assume your dosing excel. Dwarf hairgrass and quite a lot of plants don't respond well to excel. Also why are you using a bubbler that's gassing off any trace remnants that you manage to get from the liquid co2. I tried to go low tech on my 2 gallon spec growing baby tears and dwarfs hair grass, for the first few days everything seemed fine but after a week the hairgrass started turning brown and looked as if decaying,I then began dosing dry ferts and pressurized co2 and everything is growing better. If you can't afford pressurized co2 then at least make a diy reactor it's cheap and apparently very popular, I myself was going to go that route but I would rather have most of my equipment automated.