As the others have said, the first sign of CO2 issues is supping water from the surface layer.
What was the tank like that you moved them from? If it was an older and none CO2 tank, there could be differences in chemical composition of the water and pH, that plus them getting used to higher CO2 levels (even if those levels aren't enough to drive them to the surface), not to mention the environmental change (hardscape, light levels, flow changes) is likely to because them some stress. Did you drip acclimate them when you transferred them from the holding tank?
If I were you I'd stop the CO2 for the day, and turn the lights out too maybe, and give them chance to acclimatise to their new home