When I first put in a drop checker it was solid yellow and my shrimp didn't seem to be having any ill effects. I now have the spray bar aimed at the surface to give a little turbulence across the surface and it seems to be working. The drop checker is always a nice green. I leave the CO2 going 24/7 and no air stone.
What is your CO2 recipe? I am using the jello method in a 1L bottle and it is working great.
Side note, my first shrimp is berried and hopefully this tank will have lots of shrimp real soon! Now I just need to get rid of snails and some white worms on the sides of the tank (planaria?).