You probably don't have 30 ppm of CO2, but closer to 20 ppm. A .8 drop in pH means an increase in ppm of CO2 by a factor of 6.3. Assuming the CO2 ppm drops to about 3 ppm overnight, you would have about 20 ppm with the CO2 on. The drop checker isn't nearly accurate enough to tell you you have 30 ppm. At best it can tell you you have between about 15 and 45 ppm. I don't know if this is keeping the hairgrass from growing well, or not.