get 2 mollies, if that's not an option place the smallest power head you can find near the surface area and hook it to your timer so it turns on when lights are turned off, and vice versa.
Airstone adds more clutter to the tank, you need air pump, airlines etc.
Edit: although some may disagree, i never see a need to turn on/off co2 flows. Just leave it at a constant x bubble per sec for your tank, in the day plants will absorb co2, at night the tiny powerhead will create enough surface movement to balance out co2 and inject oxygen for fish.
When in doubt, go with the KISS philosophy