DC is light green. Solution hasn't been changed in awhile but reacts normally when brought into the air, etc. The needle valve is marked just before the point where shrimp start swimming around erratically, a symptom I consider a precursor of CO2 toxicity.
Several days ago I switched to a spray bar, so maybe I'm losing more CO2 to the atmosphere now. I think I'll whip up some more solution and see if the level can be boosted to compensate.